all the happy people in your life
You'll see when…
Julien Kauffmann

To be honest, I sometimes say “You’ll see when…” to my sister, but usually it’s about reassuring her that everything is going to be fine. “You’ll see when you’re over your broken heart, you’ll fall in love again. Promise.” (Teenagers…)
So there might be a positive way of saying “you’ll see when…” But then, it has to come from someone who *really* is older and/or more experienced than you. Otherwise, it’s just a way to show contempt for whatever the other person’s experience is worth.