For me, it really depends on the sim. Some of my teens are active in clubs and I set certain goals for them within those clubs for them to meet before aging up. Some of my teens are academically oriented, so I get them to the best grade possible and have them complete all the projects from parenthood perfectly before aging. Most of my teens, I wait until I max out at least 2 or 3 of the parenthood traits, either in the green or red depending on my plans for them as adults.
I tend to look at the teen life stage as I would the teenage years for humans; it’s all about building good social skills and friendships, falling in and out of love, going to school, part time jobs, and just figuring out who they want to be as adults....or in sim terms, who I want them to be as adults. It’s your last chance to set them up for success or failure as YAs, to make starting a career and finding someone to settle down with easy or challenging.
For me, that’s the fun part. Will Cassandra Goth marry her high school sweetheart, or find that her pursuit of musical perfection is more important than romance for a while? Will Malcolm Landgraab follow in his mother’s footsteps, or have a change of heart and clean up his act? Will Luna Villareal grow out of her shallow “appearances are everything” mentality, or will she become even more of a mean girl with age?
Discovering the direction I want them to take is what ultimately decides when they age up.