Yes most people assume those they see as peers are roughly their own age, and that people have had children at the same age they did.
My boss refered to me as being 35 as though she knew for a fact that was my age when I was actually 44 but not because I look 35 - it was because she knew the ages of my children and unconsciously projected (she'd had hers young, so had young teens and late primary school children by 35, it didn't occur to her I'd started having children at an age when she'd finished and had school children).
Similarly I retrained with people mostly in their 20s and a few in their 30s returning to work with children at school, and a couple in their 50s with grown-up children. I fell between stools in my 40s and the 30s people assumed I was their age because I was at the same life stage (school age children, retraining for a new career), not because of my skin or whatever.
Everyone assumes those they perceive as peers due to shared experiences are their own age, more or less.
It happens in reverse too obviously - assuming that the parents of your children's friends or colleagues at your level at work or whatever are your own age can age them significantly! It doesn't mean they look old, it's projection. Everyone does it though.
But it's all good as long as nobody is trying to corner colleagues and aquaintances into guessing their age! That's one of my pet hates because it's so awkward and puts you in a position where you're expected to lie to flatter, and if you don't you gain an enemy.