How can I work better with a younger colleague?

This has nothing to do with age

There will always be co-workers who overstate their contributions, suck up to management, try to act like supervisors, etc. You will find 65 year olds who will do this. My advice is to ignore them. I would suggest always calling them out if they make statements that diminish your work in public ("Oh really? I didn't notice.") And then don't play their pity game. But other than that just do your job. Everyone sees what they are doing, and if they don't you can't affect how bad managers manage.

There was no need to apologize. You stood up for yourself, which in my opinion is what you need to do in the workplace, because no one else will if you won't. Standing up for yourself tells this person they need to keep their comments to areas where they won't get called out over it.

Focus on the task, not the person.

Where's the manager in all of this? If it is just you two working together, then as peers, nicely ask this person to do productive work. If the manager is watching this person do nothing while you are working, that is tacit approval of the poor behavior. You will have to nudge.

What I mean by that is to let this person know provide additional help for the task(s) at hand. Getting this person to help is the desired end result, isn't it?

If they don't help, then you have specific data to note when you talk to management.

Getting "triggered" doesn't help. You understand that.

• You should not have apologized.

By apologizing you are allowing him to do the mischief once again in future. You are submitting your self as a weak person. Tomorrow he will do greater mischief.

You are also allowing the supervisor to take his side.

If you think he was wrong, call it out. Stand by your word. Be crisp. Be short. If things are not in your favor, walk out. Don't give explanations unless asked for (even to the supervisor).

I know colleagues who just call out the issue and stay by it. They don't explain. Eventually, over a period of time people don't mess with them. Sometimes they are wrong, yet.

Keep to provable facts

It is easy to say someone is not pulling their weight, but it is another thing to prove it. For example if you two are using separate point of sale (POS) then you can tell the manager they should check your respective POS to see the actual number of returns you two each processed after the boasting is done.

If there are no metrics on individual performances, then that raises other concerns. It is very easy to judge employees performance by perception and not their actual contributions. So if you can think of ways that provide hard metrics on work being done that the manager can see, then you will not need to defend your actions, and it would not matter what the coworker says they do if the raw facts do not match.

Create an equitable system that balances the work

If you are in a situation where your coworker is dumping the harder work on you unfairly, then suggest a system to your manager that will help ensure that everybody is pulling their weight.

For example if there is only one POS for processing returns then each person is assigned alternating 2 hour blocks for their shift. If a customer wants to process a return and it is 10:15 AM you both know exactly who is responsible for processing it. If returns are biased to a particular time of day, then that would have to be factored in by having alternating who gets the first 2 hour block in the shift.

I did react verbally probably in appropriately but I was so frustrated that he took credit especially when he was socializing.

The issue is not that you set the record straight. It's that your response was unprepared and off-the-cuff, and probably emotionally charged. You need to prepare yourself better, for the next time this happens (because I can guarantee you, something like this will happen again).

What I find difficult is that yesterday when he pronounced loudly he did all the returns, I got triggered as I saw him standing socializing with others when I was bombarded by customers and did all the returns. And he said to my supervisor - we did the returns - I said I did the returns, accidentally.

This is tough because there were two incidents here, and you tried to respond to both incidents with a singular unprepared quip. The first incident was him claiming that he did the returns yesterday (when you both worked on them). And the second incident was him claiming that you both did the returns today (when you're the only one who worked on them and he was too busy socializing).

It's no wonder your supervisor doesn't realize what's going on. It's also no wonder you got upset the second time around, because you were still thinking about the unaddressed incident that just happened yesterday.

I feel bad, guilty because I upset him...

Don't be. In the end, he's in charge of his own emotions. And by trying to control how he feels, you're putting him in control of your own behavior. That's not good either.

I feel so bad even when I apologized that when I stood up for the truth

In the future, be careful what you apologize for. Personally, if it had been me, I would have said: "I'm sorry for being negative, but the next time you try to take credit for my work, I will continue to set the record straight."

And to you, I'd suggest that you read "When I Say No, I Feel Guilty" by Manuel J. Smith. I know the title of the book doesn't sound like it's applicable to your situation, but I'd suggest you read its customer reviews on Amazon before you make up your mind about it.

I'd also suggest that you keep a daily journal reflecting on what happens during your workdays. You need to analyze what goes wrong and what you can do better. But this kind of introspection and self-reflection is really best done by you and noone else, and this is best done in the form of a journal if you ask me.

The main thing you need to do is be positive about it.

When you are overloaded and he us slacking, request politely for help.

• Can I have some help please?

If he doesn't then complain to management.

If you are able to do this when management is watching, that would be better.

If things are getting heated, the first thing to work on is your own reaction. When you feel irritated or blindsided, don’t fire back right away and take a breath, keep it factual, and choose the moments that are worth addressing.

And when he says something in front of others that you don’t agree with, it’s fine to correct it, just do it calmly and with some political awareness. A quick, steady “Just to be clear, I did most of those returns. Next time I could definitely use your help.” helps shape how others understand the work being done. Done the right way, it reinforces your contribution without turning the conversation into unnecessary drama.

Take a quick 1-on-1 with your manager and walk them through the work you’ve actually done, with whatever numbers or examples you can show. Bring up specific cases where your coworker described something one way but the reality was different, and limit it to tasks you both touched. Keep it about the work, not personalities. That gives your manager the context they need to reset expectations.

There are nice answers like @Tiger Guy and @chendu. You better stand for yourself and hold your ground and not fall for others' gaslighting. It even sounds like he just knows how to push your buttons and is manipulating you with his victimhood.

But I wanted to add that your reaction also sounds like a little extreme towards conflict aversion and feeling of guilt. You may benefit from a professional coach or at least some wiser friend support to help you see the situations clearly from a neutral perspective.

I'm sure that if the same thing happened to your child or another friend, you would have a very clear opinion what should the person do. While now you sound like doubting yourself because of some inner conflict.