An adult is the fourth stage of life after teenager. One of your little people can become an adult at age 18 (but they will not be able to have children until you have them adopt the house, as they are on the verge of leaving for college), but they will have preferences for Wants Children? (though these preferences change when you begin a new generation and adopt one of them).
Personality and Abilities
An adult, being very mature, has given up childish fancies as a teenager and if you drop them on most toys they will not be able to play with them (however, if you drop them at the child activity table in Virtual Families 2, the action will say "Playing quietly" while they are sitting down at a chair and will also say "Drawing" when dropped at the artist easal). They will also have a career to work on (which can be worked on at any age from the time you adopt them to their death) and advance in to gain money for your house and living. Adults are also capable of repairing bathroom leaks, getting rid of the ants, and putting out oven fires.
If the heir of the house is married, he/she will be able to have up to 6 children with their partner.
By this time, an adult has reached their tallest height, so they will not grow any more. However, once an adult reaches 45, their hair becomes gray or they begin to look older.