@Rhiannon58 I see you have solved the issue by purchasing the Young Again LTR. I'm glad you’ve gotten it worked out!
I just wanted to mention for future reference that you can also obtain the Young Again potion by using the Generations chemistry table (it builds the logic skill). There are 10 potions to discover so it can take some time to get one. But once a potion is discovered you can make it at will for a cost, and a Sim can begin using the chemistry table at the child stage. This method is useful in situations where you don't have (or don't want to spend) 70K lifetime happiness points.