I just turned 50 and there are three things I really do not understand:
Why does the US refuse to adopt the far superior metric system?
I would add women to the list but I have a better handle on women than calculus. By no means a complete understanding but certainly greater than my grasp on differential equations and their utility.
If Donald J Trump could get us on all regular time, permanently and move the US to the metric system, he would be the greatest President of my lifetime despite orange skin, a colossal combover and his propensity to make stupid tweets.