If the age gap between a couple is too large, I feel like there are significant issue that would have to be overcome. For example, the interests of the two parties will almost certainly not be the same. Then there is physical attraction. I highly doubt an 18 year old is physically attracted to someone who is 65.
If you are a teen and you are dating a 40 year old, it matters if the relationship becomes sexual, because if anyone found out, that would be charge of rape. As long as the age is an acceptable amount difference between a sexual couple, then it is totally fine.
You can't choose who you love, or when you fall in love. Or how. It just happens. Trust me, I bet a few people wish they could stop loving someone, for multiple reasons. You fall in love with personality, common interests and attraction. Though attraction may fall out of the factor with older ages, it's still a possibility. Honestly, love is love and if you don't like it, don't get involved in it or with people who are because it's not of anyone's business
While age may play a factor when it comes to attraction, it is not a prerequisite for a healthy relationship. Many couples of distant ages have gone on to live happy lives and lead strong families. The heart wants what the heart wants and in the face of true love age will never be a factor.