My boss is being discriminatory towards the younger workers in my department.

Hello, I am coming here looking for any sort of advice. Me and my friend both 20f are being treated differently at work by our boss. The rest of the staff are 30+ but mainly 40-60s. There are around ten of us total. Recently we’ve been being spoken to like shit compared to everyone else. Last week he asked me to fill my trolley and I said I am finished. Because I told him no he got really rude with me and threatened to take me to the office for a disciplinary which I was very confused on how he had decided to take this drastic action but I stood my ground and I said it is already 5 minutes past my finishing time so I will be going now. I presume he thought that because I was younger that I would just give into what he was saying because he would never dare speak or even ask any of the older folks to do that (I spoke to the older ones and they even said they leave their trolleys unfilled if they are finished so it is definitely extremely common for them not to be filled). Ever since that day he has been nit picking and only giving me and my friend into trouble. For example a couple of days ago me and my friend took 20 mins on our break instead of 15 mins because the queue for the till was quite a bit longer and we had to wait. When we came back he waiting until it was just us and gave us into trouble which would be fair enough however nearly every single person in our department are known for taking breaks longer than they’re supposed to but it was only us he has said anything to. When we explained why we were a little longer his response was “yous are grown ups maybe you should start acting like it”? He also makes comments to us telling us we are not working fast enough etc(we are literally the fastest in our department our speed rates are put on a board where we can see them so this is a fact). It’s not really too serious I suppose but I used to enjoy my job and honestly since he has started doing this it has made it miserable just because it’s only the younger ones who are being singled out this way. I cannot express enough how he would never speak to any of the older ones the way he does to us even though we are only 2 years younger than him it honestly feels like being in primary school. Even the older ones have brought it up that they do not get treated the way we do. I genuinely would understand if we actually were not performing how we should and were rude, lazy etc. but we pass the store expectations by a lot, we just want to come to work and be treated the same way that everyone else is. I would also like to note that I have only started standing my ground the past couple of weeks and I think that may be why he is being so harsh all of a sudden.