Board Thread:Theories and Speculation/@comment-31266380-20170213121035/@comment-3965545-20170303200328

Considering they were in the middle of the third war for about 4 years before Naruto was born, and Minato was busy fighting most all the time and doing Jonin missions and duties.. he could not have had time to go to Mount Myoboku learn Sage Mode during the war, or as a Kage.. so he had to have learned Sage Mode before Kakashi was 10yrs old(he became a Jonin sensei to Obito, Rin and Kakashi when they were 9) therefor he had to have learned Sage Mode at the latest right before the 3rd War Began.. meaning he had it for 4+ years before he died. So he just isn't as skilled with Sage Mode techs and usage, as others are.. that and he simply did not have the time to master it fully like Naruto and Hashirama did. If he had time I am sure he would have been able to master it.