Well, my problem is that if I DID stay home until my youngest is 18, I'd be 57 when attempting to return to work. Because I work in education (& because we're not DESPERATE for teachers or librarians here), I'm pretty certain the type of job I'd have to take would end up not being worth it based on paycheck alone.
I did return to work at 45-ish after "only" working part-time as an adjunct for our homeschool years and I definitely think age discrimination is a thing!