Hey, I can relate to this situation in one way or another, however my friend was only 4 years older than i was. Regardless, I will give you my story, and hope it helps,
Sicne we shared mutual friends, he told them before he told me, but I couldn already tell, as most people can when someone is interested. Our friends told me, because they felt i should know. When he told me himself I kindly told him, that he is one of my closest friends, and i think its a phase, and i will treat him no differently than i have, until it goes away, and we will remain friends.
I turned him down everytime he made any sort of move, and was sure not to do anythign that would lead him in the wrong direction.
This lasted for 3 years… it even interfered with my own personal life, because I was involved with a mutual friend of ours and we didnt know how to tell him, so we kept it a secret. Until only recently when we decided that our feelings for each other were much stronger than had ever expected to getting, and we decided to date. We kept it to ourselves for the first couple days, until my bf had a chance to talk to him and explain to him the situation. He said he was okay with it, but we both knew it bothered him, it took about a month, but he came around, and told us that initially it bothered him but hes happy for us now, and has since got a gf.
If you value the friendship i’d try to find a way to salvage the friendship, but i do not want to see you go through what i did. I felt guilty for being with the one I loved, even though I didn’t ahve feelings for him and made it clear.