I don't know of any particular chat groups for younger gay men Med. One piece of advice I'd offer you though in seeking out such chat groups for younger gays, is to be very careful not to get "catfished" by imposters online. Some people may only be posing as younger men to become intimate with guys who are 18-25 years old for instance. They try to gain your trust, ask you for all the intimate details about your body, your sex life, what you like to do sexually, etc. They tell you how young and beautiful they are. Then they might start asking you for nude pics of yourself with the promise that you'll get some pics of them in return.
Then reality sets in. After sending them nude pics of yourself, the promised pics of them either never materialize at all, or the pics you do get from them are all fakes. Then you come to figure out that this person posing as someone who is 18, 20, 22 years old is actually some guy between 40-80 years old...who is out trying to scam and sexually harass younger guys online for his own amusement. I'm not saying that there aren't good, honest people in the world you can trust... Or that there are not wonderful, honest people online. But online one has to be especially careful about imposters who might be there for their own selfish ulterior motives and agendas...that have little to do with respecting you as a person.
So just be careful out there. Don't get your heart broken unnecessarily.