After you travel to a WA location the first time it kind of pushes the start button for that location. Sims living in France, for example, don't seem to age while your Sim is there on vacation (at least I never seen it happen) but after your Sim goes home Sims in France do age. They seem to age faster than the the Sims in your Sim's hometown. There's a bug with the relationship panel though, where WA Sims don't look older even if they have aged up. Their pictures only get updated the next time your Sim sees them. Advice: don't wait for his picture to look like he has become a YA to invite him over! In my Dynasty I sent a child Sim to France (with adults obviously) and she met all the children and teens there. When she aged up into a teen I had her invite her French friends over and they were all adults now! Even the ones that were children while she was in France and "should" be teens like her. Most likely, when you invite the guy from France to visit, he will be an adult and you will need an adult in your household to ask him to move in (if your Sim is a teen).
I've never had a problem with the WA Sims not aging, my problem has always been that they age too fast!