>>96888912
I feel like her existing as an adult implies one of 3 things.
1. The reality she comes is an alternate dimension where everything Jace believes is good happens so Jace and Vraska met as teenagers and started a family early on.
2. She's from the future. She is either trying to prevent her existence or insure her existence. According to Strixhaven lore the Archaics are Strixhaven Oracles who are sent back in time as Archaics with all their knowledge and reality changes to make it seem as if they always existed. This could be why she's at Strixhaven.
3.Lorwyn will begin another huge twenty year time skip. The Lorwyn natives would aged 60 years but elves are the only species with a confirmed lifespan of 40-50 years so maybe the others can live to 100 making Brigids appearance appropriate.

Look at me, I'm now the spellslinger deck.