They're not just being nice, they also fantasise about having a son like you, but really, it's a sexual thought.
It's kind of like being a 40 YO guy with a teenage daughter, and your daughter invites her cutie friends around for a sleepover and your heart races when you see them joking around, being all playful and smiley. Even at 27 I look at girls younger than me and think "gosh damn she's cute, wish I had a daughter like her" and yeah it's always a sexual feeling but it's also because as you get older, you are reminded of your youth whenever you see other young people.