I waited long enough, in my book. I was raring to TTC for about two years (so, almost as soon as DH and I were engaged) but made myself a promise that I'd wait until after the wedding. About a month before the wedding, we figured we might as well start trying and if it worked we could pass it off as a honeymoon baby... The next day (or so) I found out I was already 4ish weeks

ETA: Considering I started getting clucky around babies at age 12, I think I did alright Ianto was born when I was 23, Amelia when I was 24.