Yeah, the appropriate response here is to go 'Oh no, they grew up while I was busy with other stuff! What do I need to teach them about that I haven't?" and give them any part of the safe sex, consent, internet safety, ykinmkato, and how to get help with bad relationships talk that you missed till now.

(It's okay if it's awkward as long as it's kind, if you hit the awkwardness perigee in a conversation you initiate it'll make any conversations they need to initiate with you later seem easy in comparison.)