I've finished all the quests and so I though maybe I'd start a little mini series on life in Sim town.
This instalment will be one building friendships to the point of relationships and eventually marriage.
I always use to have best friends become romantically involved, then I got bored of the extra time it required and then I started beginning the dating process from the Good Friend stage. While it doesn't affect how fast the relationship builds, it does, it seems affect how effective the proposal is. I use to never have any issues getting my Sims engaged with the 3 LP ring. Now it requires the higher ones. With the 10 LP ring you're basically gaureteed a 'yes'.
So I highly recommend that you take the extra time to build the best friend relationship first.
Teenagers can build romantic relationships too. However, they take a long time and once they age up they are in a good friend status relationship, but you can rebuild to the best friend stays and then the varrious levels of romantic. You can earn some serious XP bonuses by spending the extra time though.
Well, happy Simming and good luck!