Not anymore! The StayInLove hack retains teenage love and crushes after a Sim grows to adulthood.
Teens turning to adult, though still fall out of love. Freetime allows you to grow three townie friends along with your Sim when he/she transitions to the next age. When Freetime came out, I had big hopes that it would change this.
It's debatable whether that's more or less realistic. I made this hack because I never really liked it that Sim teens would grow up and fall out of love with their girlfriends.
