Basically, I think you need to be over 30 to know how long forever is...
If you want to get married, do it. If you get sick of eachother, get a divorce before you hate eachother. Most marriages fail. You get married to enjoy more out of life by sharing that life with another person. When that doesn't happen it's time to be adult, give eachother a hug and walk away. No fighting! Forget marriage counselors. Being married is either real good, or real bad. If you love someone, there are never any OK days. Days with that person are either good or bad. You can't counsel that. Better than a marriage counselor is a good friend who will tell you to your face when you're being a bonehead and help you keep a good perspective on everything. Someone who's been there, done that. Not a single friend. Not a friend of the opposite sex.
The best perspective on marriage that I have ever heard is when someone asked if they still had the same friends they had in high school. If not, why would you expect to want to be married to the same person, 5 or 10 years from now?
So, if your expectations and your spouses are reasonable and mostly compatible, then you probably stand a chance of being married for a long time. In 4+ years of marriage, adapting to eachothers expectations is probably one of the most difficult, hurdles, and most gratifying when you both accept how it goes. (Probably have to be there to understand that one, although you guys may have worked through some of that after 7+ years of dating.)
Another suggestion - being younger, determine two things before you get married. How will you accumulate assets? Indivually or mutually? My suggestion would be don't mix your finances up. Manage your own money, not eachothers. That way, no one can feel like they are being taken advantage of. They say money is the root of all evil, so keep that evil out of the marriage equation. Then, get a pre-nup which determines how anything acquired after your wedding day will be distributed should your marriage end. I know, it's not romantic to sign a pre-nup! So, sign it over a candlelight dinner. If you're young and don't have anything except bills - guess what? If you get married without determining the above, you'll wish you had if you're divorced with half and bigger bills.
Marriage is great. But it's an adult institution which requires planning and thought and consideration for the other person. There really isn't a magic age.
If you or he has the slightest thought about waiting, then wait. Neither should be married to someone with an ounce of doubt. It's not fair to the other person.