Hmm I'm not sure I agree with that. From my experience the period where family is most likely to be a problem with a demanding job is when kids are very young. The short nights, demanding daycare schedules, having to clean them, feed them, and provide a mostly continuous watch, etc. It's pretty gruesome. If I look around me this tend to happen around 28-30 years old.
Of course family remains #1 priority afterwards, but there is enough flexibility to go along with a demanding job (provided you like said job).
I've been working remotely for the larger part of may "young father years" so I guess this helped a lot, too.
Where I have lived, the age of getting children has been getting higher and higher. It's not unusual for couples to have their firstborn in their mid-30's.
I've been in interviews where they made it so obvious they didn't want anyone with any commitments it was cringe-worthy. Also accompanied by a contract in which you agree to do as much unpaid overtime whenever deemed necessary by the company in order "to do your job".
Given that the IT industry is notorious for long hours and impromptu overtime it's easier to see why they might be prefer younger employees.