>At what age do girls start having orgasms?
They have the potential before birth. Practically speaking, it's probably going to be around age 1 when they have the dexterity to be able to stimulate themselves.
>When do they start to squirt?
Girls can pee from birth as well; no reason it can't happen anytime they have an orgasm. If you're talking about "real" female ejaculation, nobody can even agree it actually exists, so you're basically asking about the anatomy of Bigfoot here.