I left the US in my 20's to live in Europe and I like raising kids here (I'm now in my 40's) but honestly... it was a stupid thing to do. Meanwhile, my in-laws, who stayed in the US, are now retiring to France in their 40's with a few million dollars.

The opportunity cost of living in Europe is absolutely enormous. You're basically lighting $1-2 million (or more, perhaps) on fire for every decade you're here instead of the US.

Though I don't find myself wanting to move back to California any time soon...