Is it decidable if a TM takes at least 2016 steps on all inputs?
A Turing machine only sees (at most) the first 2015 symbols of the input in its first 2015 steps. Hence whether it stops within 2015 steps depends only on the first 2015 symbols of the input. This gives an algorithm for deciding $L_1$: run the input machine $M$ on all inputs of length at most 2015, and check whether any of them stops within 2015 steps. If so, reject. Otherwise, accept.