M.L.
I also didn't reach that height/weight requirement until I was a late teenager... really I'm only 2 inches and 10 lbs from it now. Even in drivers ed, they made me sit on two phone books.
I will go with the age of 8 years old, but also will get a seatbelt positioner or tightener. If my children are extremely small, then yes I will wait a few more years since it's for their own safety. The height/weight requirements is a recommendation, but the age of 8 isn't just about size, but also bone strength and such. If the children are that small, safest place for them is in the back seat, even if they are older.