There was 15 years between me and my H when we met and married. I was 28, he was 43. At the time it felt completely unimportant. However his health did start to go downhill and there were many occasions when we attended hospital appointments together and were greeted with comments like "It's lovely that your daughter's come along today" which was always embarrassing (for him, I didn't give a shit.)
We then split up (nothing to do with age gap) and he went on to have a relationship with a woman 25 years his junior. They were together about 2 years, I don't think particularly happy from what our son told me, until he unexpectedly died.
So his last girlfriend was coping at age 25 with losing her life partner, and as he was still married to me there were legal complications around the funeral, etc. He had no assets but he left her with a load of credit card debts.
Having said all that, I truly believe that if this man makes you happy, you should stay with him. None of us knows what the future will bring. He could live another 20 years, or he could go tomorrow. Equally, so could you, so could any of us. So when we find someone that brings us love and happiness, I think we should take it.
Just protect yourselves financially around wills, insurance (if possible), etc. I wish you every happiness 