I always figure out personality and culture first, because my characters' ages are used to complement them.
My characters are almost always retools of existing OCs, so everyone already has a culture they belong to. Humans in my universe are often pressured to "grow up" and become independent much faster than the humans of our world, so a lot of my human characters are in their mid-to-late teens even when they act much more mature. (I'll often age them up slightly if the culture I'm shoving them into for the RP is significantly different, or the age limit in the CS calls for it.) It's also unusual for these humans to live past 60, so 30s or 40s is considered "old" and I only go that far if it indicates or complements something about their character I already had down. (They're immortal, they've seen a lot of shit, they have outdated mannerisms, etc.)
Someone of a nonhuman species calls for different rules. I try not to do any direct scaling (species A lives to be 50, and species B lives to be 100, so someone from species A at age 25 has the same maturity as someone from species B at age 50, etc.) since it doesn't make any sense; it's more about how fast the species matures rather than how long they live. That's why there's no easy way to accurately calculate your dog's age in "dog years"; it varies by size of the breed and how far they are into their lifetime.
Generally I don't have any limits on character age, but the only children I play (~ 10 or younger) tend to follow certain
horror tropes. c:
As for birthday, I don't have any specific values down already because I don't care. If a CS calls for it, I'll usually start with whatever astrological sign fits them the best (lately I've been running some of my chars through a personality test to decide for me) and pick a random date in that range. So it changes a lot.