I was thinking with Jirachi is technically he spends most of his time asleep but because of StarFall being a massive event for Legendaries celebi probably goes and pulls him out of his sleep and gets him to come.
I can imagine he is a small sleepy boy for the most part, but the dimention thay StarFall takes place in has an energizing Aura that means whenever a Legendary is there they have no requirement to use their abilities if they need; like moved that need to be charged dont need to be charged and can be used. And for Legendaries like Jirachi the wish ability doesnt need the comet or the 1000 year slumber.

As for the Ages of the legendaries Arceus is as old as time, same probably with the creation trio, the legendary beasts would be as old as the burning of the bell tower so probably only in their first few hundreds of years. Gavin and Niccia are fairly young because Arceus has made copies off them before so theyre technically not 'original', the same would probably for Jirachi, Celebi and Victini; they've probably used up their energy too much in the past so Arceus has needed to remake them.
Regigias i can imagine is very old, the titans would be like his children so they could be hundred or so years younger than him. The weather trio would technically be old too; but in their human forms they look between 18-40 depending on the legendary; the smaller mythicals(Victini, Jirachi, Celebi etc) would look in their 18-20 age, somce they're seen as 'babies' to the legendaries.
Arceus will probably look like hes in his late 40s maybe early 50s, Giratina always looks around 30 cause she wants to look pretty for Yveltal.
If you'd like I can do an age cheatsheet once I'm back on my pc after work for all the ages?