Yes
Like every other level based evolution, Sliggoo will try to evolve (given the right conditions, i.e. rain in the over-world) at every level starting from 50.
It will begin to evolve at 51 in Route 17, and at 52, 53, and so on till 99, should you try and stop it every time. Only level 100 Pokemon can not evolve by leveling up.
Source: Experience