I have a feeling the ending is going to be more than just slightly ambiguous.
....how old is Richard, when did he get there, and why he was made ageless....
From the S5 finale... [WARNING SPOILERS!]
when we saw the (presumably) Black Rock sailing in the distance from the POV of Jacob and his buddy, as well as Richard being referred to as "Ricardo," I have a feeling that Richard perhaps came from the Black Rock however many hundred years ago and for some reason has been charged/punished with serving the island. Why? you may ask. Inference really. Richard/Ricardo is hispanic, and perhaps the Black Rock was a Spanish boat of some kind? Whatever transpired with him landed him the ageless roll of the island leaders' advisor.