So, I have a sylesti in my hatchery, he's an adolescent and I was just wondering when he would turn into an adult and when do hatchlings turn into adolescents?

Eggling stage lasts 3 days/ 72 hours (locked at 72 maturity hours until hatched, autohatch after 24h form first reaching 72h) (starts: 0h of Bred/ Bought, random if found while exploring)
> skips forwards one hour per every 6 correct nurtures

Hatchling lasts 6d / 144 h (starts: 72h)
> skips forwards one hour per every 4 correct nurtures

Adolescent lasts 18d / 432 h (starts: 216h)
> If given a Magical Pie, the pet will gain an additional 144 Maturity Hours (6 days). As there is a three day cooldown on Magical Pie use on a pet, you can use a total of two pies per pet (leaving you with a total wait time of 144 hours/6 days if you somehow manage to time it perfectly (and don't forget)).

Adulthood starts at 27d / 648 h