What age do Sylvesti become adults, and how do you breed someone else's pet with yours?? Also, can you breed adolescents?
I know you can make your pets adults using magic pies, but I don't have any gems to buy them with. Any other ways?

Sylesties become adults at 648 maturity hours, or about 27 days.
Pets can be set up for breeding via a fee, and can be searched for in advanced search, where you can click on the price option and breed them to your pet from there.
Magical pies can be obtained from various festival activities. or purchased from the trade broker for gold.
Adolescents can not be bred, that would be weird

Thank you so much!!

additional note:
27 days would be the time for a newly bred eggling or created exclusive eggling, if it receives no nurturing at all.
Maturity hours are the "timer" that counts to when a pet reaches the next life stage, rather than their age in real time, and is affected by different things (nurtures for eggling+hatchlings, pies for adolescents.)

A pet hits the start of adolescence at 216mh (if made via the generator, this is where they start life, while for a bred egg, they'll be nine days old at max when they reach this point), and takes them 18 days from there to reach 648mh if no magical pies are used to accelerate growth.

Wild caught sylesti can potentially have any maturity counter age between 216 and 648 (with 648 being any pet that is adult), and you cannot see the exact mh count unless the pet is genetically tested.
(you can see the mh counter on generated pets and starters by default, however, as they count as being kinda,,, auto-tested? by the game)

Thanks for the additional info!