Though Alexander Grothendieck is considered one of the greatest mathematicians ever and I was aware of his name and influence from 1966 or so, I was completely untouched by his work. One great mathematician of recent years is Misha Gromov. I tried to learn a little bit of his work after the age of fifty and even wrote an expository article with Martin Bridosn (M. R. Bridson and G. A. Swarup, On Hausdorff-Gromov convergence and a theorem of F. Paulin, Enseign. Math. 40 (1994), 267-289 file:///C:/Users/swarup/
Downloads/ensmat-001_1994_40_ 3-4_a_004_d%20(1).pdf , I am not sure whether it works) on one of his ideas. Here is a small article about him. An interview with Gromov.