21 year old dating 26 year old
I think it's important to realise that a lot of things that may seem trivial or stupid to you (the older party) are going to be a big deal for your younger other half.
You can have things in common, but that doesn't matter if she or you can't handle a relationship without possessiveness.
The best advice I could give you, personally, is that her parents may be somewhat disapproving and it is very important that you respect their concerns.
My friend is 30 and in a relationship with a 21 year old.
She is going to be in a very different stage in life, and for a while there will be a very distinct imbalance to your relationship.
That's not necessarily the end of the world, but you have to be a little more vigilant to make sure your relationship is always one between equals and neither of you ends up resenting the other. I am in a relationship that started with her at 19 me at 25 we have been together 7 years at this point.