Fisher Tiger was mentioned most likely to be a fish man since: a) he hate human and b) when it comes to slave he indiscriminate all races (so when it do not come to slave he do discriminate). But he could be some other race as well since he was not specifically mentioned.
Back to Boa's age (ok pure discussion about her age, leave the "mother" part aside)
I agree with SlayerX that the conversation do not prove Boa to be 36 or older.
From the day Hachi save Railegh to when he join the Sun Pirates could be ranged from 1 year to 20 years. Then the "20 years" Rayleigh mentioned could also be an approximate number (to be various from 17-22). Without considering the time take to travel between islands, we knows Boa can be from 17 to mid 40 (which is not helpful at all).
Then we don't even knows if the Sun Pirate was created after Fisher Tiger saved the fishman or not. He could have a small unknown pirates group(include Hachi) named whatever it is. And only be known as the Pirates of the Sun after he saved the slaves. Just like how Luffy has traveled around the sea for a long while until he and his crew were known as Straw Hat Pirate.