My younger daughter's partner is 23 years older than her, and they have been together since she was 18. She is now 30. They have 2 lovely children (he has 2 adult children from his first marriage) and are very happy. The age gap has never been a problem, except for her worries about his health, as he has Type 1 diabetes, and her fears of being a widow when she is comparatively young. However, this is a risk you have to accept if you are in a relationship with someone quite a bit older than yourself.
As for the feelings of others...Is it really anyone else's business? Of course, as parents we have emotional opinions regarding everything that involves our kids, but we have no say over their relationships when they reach adulthood. Maybe we wouldn't have chosen an older man for our daughter, but it wasn't our choice, and DD's partner has good qualities which are far more important than his age. I know, from personal experience, that interfering in your children's relationship only leads to damaging your own relationship with them. My DH is Indian and I am English and his mum and dad did everything they could to prevent us marrying. They only succeeding in driving us closer together and his relationship with them, particularly his dad, suffered in the process.
Basically, like it or not, it is up to them to choose, not you. All you can do is be there for them if they need you.