Anyone know how I grow a pet??

Pets age over time.
A pet is an Eggling from 0 to 72mh (3 days total) (mh = maturity hours)
Hatchling is 73 to 215 (6 days total)
Adolescent is 216 to 647 (18 days total)
When a pet hits 648mh old (27 days), they're an adult and can breed.
(* the 'days' is relevant only to pets that start as newly bred or created eggs, and to give a better idea of how long each period is, see more below)

You can see a pet's MH if the pet was Generated, or has been genetically tested (people offer testing services in this subforum for people who don't have an infinite gene testing kit)

If you look at your starter's info, for example:
Birthday: 03-08-2023 2:55:13PM
Age: 7 Days
Maturity: Adolescent (387/648)

That last bit, that we've bolded, is her mh number, meaning with some quick maths of 648-387 we can learn she has 261 hours (~10-11 days) until she's an adult


MH is not necessarily correlated with 'time the pet has existed', as wild pets/found egglings/found hatchlings can be any age in their mh bracket, generated pets start life at the start of adolescence, and mh growth can be sped up in various ways

Egglings and Hatchlings can have their mh accelerated with Nurtures
(egglings get +1 to their mh for every 6 correct nurtures,
hatchlings get +1 mh for every 4 (to balance the +1 to the lowest stat that they get every 8)

Adolescents can be accelerated using a magical pie (+144 to their mh, three day (72 hour) cooldown after one is used, so you can use a total of two pies to skip 288 mh total (12 days)

Thanks so much!! Thats super helpful!