Was Justine a slayer in the "Buffy the Vampire Slayer's" spin off mostrar "Angel"? If not, then how was she that strong, and if she was, why was she so much older than any of the other slayers?

A) Perhaps this is a better pregunta for the ángel spot as (though I think it's a real shame) not every Buffy fan is a fan of ángel the Series, and have no idea who Justine is o what you're talking about.

B) I recently rewatched ángel last año and I don't recall Justine being supernaturally strong. She trained a lot with Holtz, but she was fueled por revenge, not Slayer power.

C) Age is no indication of whether o not you're a slayer. The only reason there aren't a lot of older slayers is because slayers are normally called when they are teens, and generally have abreviated lifespans as a result. If anything, an older slayer (and por "old" I really don't mean so old - but like 30s o 40s) should probably command some respect. After all, it would mostrar that she's survived that long. I like to think of Buffy in her 40s, being such a strong slayer, even at middle age.

No, I don't believe that she was a slayer. She was probably just strong because she trained after her sister's death.
As for the age thing, even though it seems like most of the slayers were called in their mid to late teens, I dont believe there was any hard and fast rule for how young o old they had to be.

I like to think Justine was a 'half-slayer'. After her sister died at the hands of vampires, she was 'called', due to the Slayer dying at the end of series 5. However, because Buffy had already died once before (and as a result meant that Kendra was called), her segundo death only gave Justine some, but not all of the usual Slayer power & attributes.

Works for me, anyway!