I start my Sims' families when they are YAs to help maximize the time. I also play on LONG and age my wee ones up when I'm good and ready to do so. I found out the hard way, in one of my saves, though, that Long isn't the same as eternity. :s I was waiting for my Global Superstar to get to the top before I aged him to YA. Since he had two younger siblings that I wished to space adequately, by the time his only sister was where she needed to be (YA) and had met her future husband, I got a notice her new husband's mother's spirit was fading away. (I didn't even know she had passed!) Not terribly long afterward I did get the notice his father had passed. Ho-hum, neither got to meet their grandchildren. :'(
Such as life, simulated …
To answer your question more directly, I would guess that you start your Sims' family when you're ready for it. If all else fails, turn aging off.