I think sometimes people have a bit of a shock when they suddenly see someone they've always known as a kid as an adult. As a father, there's been a few of them for me. Sometimes its something about their personality, like the first time they legitimately out argue you on something, or know more about something than you.
And sometimes it is physical. For me that was seeing DD in her prom dress at 16 and thinking "Bloody hell, when did she turn into a woman?" And yes, part of that was that she suddenly had cleavage, and a waist. (She lived in baggy T-shirts at the time). And no, that doesn't make me a perv for noticing.
And it works the other way around as well. I can think of at least two occasions when my Mum probably had that "He's a man" revelation about me. The first was when I legged it from my bedroom to the bathroom naked to chuck something in the wash basket and timed it badly, giving my Mum a view of my backside as I ran back into my room. I know she got a good view because I got a "When the hell did you get a hairy arse!" shouted after me.
The second time was the summer between 6th form and uni and I'd been swimming lots and started going to the gym. I turned from a weedy little nerd into a, well, somewhat less weedy little nerd. I took my shirt off down the beach on holiday and got a "You've got shoulder muscles!" from her.
Was she perving on either occasion? God no, just realising that her kid wasn't exactly a kid any more.