@harmonica,
If the relationship is good, and both of you are getting something out of it, why do you care if it is weird?
The only real issue I see you mention is that dating her makes you feel old. That doesn't seem like a huge issue to me.
Jespah mentions being in different stages of life. I am not even sure that is true without knowing more about your lives. There are 36 year olds in college and 21 year olds running businesses. Even if this is true, I dont see why it would matter.
The real question is what you want from this relationship. If one of you is looking for marriage and kids and the other one is looking to just have a fun relationship then there may be an issue.
But if you have a good relationship, your meeting each other's needs, and you both want the same thing from the relationship, then what's the big deal.
If the relationship is working keep it. Finding someone isn't always easy and there's enough to worry about in life already. If you've got something good going stop worrying and just enjoy it.