I have a 19 year old dd (20 in 2 weeks) who is seeing an older man - he is 33. They have been seeing each other for around 6 months but things have gotten a little more serious over the last couple of months - before that it was just really more of a seeing each other occasionally thing. I know that he has struggled with his feelings towards her because of her age - he was quite open with me about that when we all went out for a meal and a couple of drinks together about 4 months ago. Their friends don't seem to have any concerns about an age gap - it just is what it is.
They have known each other for around 3 years as she used to work for him and then met up again just before xmas when she was out in town. I hadn't met him but knew of him as he is a relative (by marriage) of one of my work colleagues.
We have now met him a few times and my dh and I do like him (rather begrudgingly admitted by dh as he expected to hate him). He treats her nicely and they do seem to get on really well. They have joint friends of all ages and also have their own separate friends. They do spend a reasonable amount of time apart as they both have their own interests (he football, she her horse). She does tend to stay over at his more than she is at home at the moment. He has just been away on a lads holiday and she is off on a girls holiday next month. There seems to be no possessiveness in the relationship - which would definitely ring alarm bells with me. They give each other space and are just taking things slowly from what I can gather from dd. (I don't ask about the relationship, she does tend to volunteer information if she wants me to know).
Whilst I think life would be easier if she had a partner who was a similar age I don't have a huge hangup over it. My sister's partner is 20 years older than her (she 52, he 72 - they have been together around 28 years), one of my friends is 55 and her husband 70 and a couple we see in the pub got together when she was 18 and he 31 - which was at least 20 years ago.
I think the main thing is to keep communication open. Invite him round for dinner or go out for a meal. Don't make him into something that she has to be secretive about - it will make him much more appealing!
If its going to turn into a long term relationship it will, if it isn't then it wont but keep him as your friend and not an enemy.