If we can get to the point where we're reversing aging, I think we can safely assume reversible sterilization.

Use an economic solution (really simplified here): When you receive anti-aging treatment, you're temporarily sterilized. Each person then gets 1 credit for a child. 3 credits are required to bear a child. One can purchase or sell their credit for children as one would like.

For the first time in human history, if one wants children, they'll have to put conscious effort into it. Only those who _really_ want children will have them.

...unless you don't want anti-aging treatment, then you can have as many kids as you'd like and become 'immortal' the old fashioned way.