Ah. Youth potion is perfect. It will get them closer in age and then when she ages up to an adult, He can join her. I wouldn't have to do this if they somehow made it so when you can "get to know" a person, you can find out how old they are. Thanks guys! Note that I have all EPs and SPs.
Question: I did find the "youth potion" in the happiness rewards. Does anyone know if someone can purchase that with their rewards and give it to someone else?
EDIT: Yes you can give the youth potion as a gift to someone. BUT, be aware of this: my elder Sim is the only one with enough points to get one. He gave it to the heir by just putting it into her inventory. She gave it as a gift to her fiance before they got married. Once they got married, I checked his inventory and it wasn't there. So reloaded and did the process again. This time, she gave it to him after they got married. That time it was there.