I started young, so my cut off age was always 30. I don't know if I had had difficulty with falling pregnant if I would have stuck with that cut off, but I never had any interest on having a large gap between my kids so the idea of freezing eggs or embryos for down the track has never crossed my mind.
As a previous pp, I knew I was done when I wasn't sad getting ready of the baby stuff, and we also can't afford any more kids with our chosen lifestyle. Neither of us were prepared to give up our lifestyle for more kids so that really lets you know that you're done...plus the downside to have kids close in age when they are all young is the difficulty of raising them all when they're all at super demanding ages. All my kids are primary school aged now and I love the freedom and independence this stage has given us as parents. My husband has had a vasectomy, and we have no regrets.