His age is not mentioned. Phineas, along with his stepbrother Ferb Fletcher , star in the A-Plot of every episode. The series concerns Phineas's attempts to avoid boredom by finding something new to do every day of the summer vacation. He does this with his less-talkative stepbrother Ferb,  and often with many other neighborhood children. He is known to be very selfless and energetic. Phineas has many catch phrases like "Oh, there you are, Perry" or "Hey, Ferb!
Isabella Garcia-Shapiro - Works | Archive of Our Own
Maybe a better question is; when did I realize she was the love of my life? Maybe it was when I caught her coming out of the shower. Ya, that's probably it. Now if I get something wrong please let me know and I'll fix it, but don't be too mad about it, because I am sadly a virgin myself and I'm going off of what other people have written, so other than that. I lay awake in my bed, just thinking to myself.
Creeply Isabella is alone at her first week of college. Whatever is she to do without Phineas? Warning heavy lemon, hard smut, Out of character.
Edit There is plenty of evidence that Isabella is at least part Jewish. Her last name is Shapiro, traditionally a Jewish name. She has a mirror in her room that has a Star of David frame; this is a common symbol for Judaism. She traveled through a floating eyeball as the good witch and her friends were the Patchkins. The character is named after series creator Dan Povenmire's oldest daughter who loves the show and enjoys that Isabella is named for her.