Honestly, in my opinion, if you ever plan to actaully be with someone long-term, as in possible get married, move in together, actually structure a life together, then you better wait until you're reaching 30 or over.
There are exceptions, of course, but from my expeirence in dating, from my expeirence in talking to others and witnessing other relationships, you should always take a relationship in your 20s with a grain of salt. Nobody knows what they want in their 20s, you don't even know what's out there. Getting emotionally invovled with someone who has no idea what they want, will almost always lead to something sour.
My advice for pretty much everyone? Have fun in your 20s, get a lot of tail, explore with different women and get out there, party some, get some experience, learn what is really important in a person, then when you're in late 20s/early 30s, you will have had enough partying, enough women, and will be ready to find someone who is in the exact same state as you, in their 30s, wanting to settle down and build a life, someone who has been there and done that and is ready to calm down and start a solid, structured life with someone.
There are exceptions as I said earier, this isn't a 100% full proof plan, but it's definitely my adivce and the best plan that a person can make judging from my experience/the experience I have witnessed from others.
So, I agree with Xeb, for same reasons and others. Unless of course you're already 30+, and if you are, you will want to find someone who never says "I need space", that's just retarded. Find someone who won't do some stupid teenage crap like that.
"You lack the requisite spine and testicular fortitude to study under me"