Both Page and Brin turned 40 this year. There's also Brin's heightened risk of Parkinson's. His mother has it, and according to Wikipedia "both he and his mother possess a mutation of the LRRK2 gene (G2019S) that puts the likelihood of his developing Parkinson's in later years between 20 and 80%". He's donated to Parkinson's research in the past, and this investment is certainly along the same lines.