Or you have your kids early, raise them, and then focus on community tasks (like running the ship) after that.

I don't think I regarded my kind, wise, and friendly grandmother as a 'non-reproducing adult resource drain'... Seems like a cruel way to describe one's golden years.

It is definitely a cruel way to think of it, but the thing is that in a generation ship, you have resources for a limited number of human beings. Longer lifespans mean you have more human beings to support. Life extension makes more sense if you can have the kids at age 50 to keep total population within reasonable limits. On a generation ship, it makes more sense to have a Logan’s Run scenario where once people reach a certain age… Carousel!

It seems like people being able to reproduce at-will would be the problem. Designing a system where people are sterile by default and can only have a child with intervention seems the only way to prevent random population explosions. If some group in your population finds religion and decides to be fruitful, you have a big problem. If 15% of your population has double replacement numbers, you have a 45% increase in your population in two generations.

There are definitely some huge social challenges in the generation ship concept.