Larry got it right, but the solution really doesn't require removing socks to count toes! :-)
Jane was born in 1897
Every year after that, she celebrated her birthday on the same day, and in 1918, on her 21st birthday, Ben was born. (If you want to consider the same day of the year to be the first Sunday, you might be up to 6 days off from the actual birthday, but that is a small enough error not to affect the final answer.)
So Jan was 21 years older than Ben (and the actual dates do not matter)
A little algebraic notation: For any year date x, Jane's age j(x) = Ben's age b(x) + 21.
For the date when Jane was 5 times as old as Ben, we can write
5*b(x) = b(x) +21, and solve for Ben's age as 5 1/4.
Six years before that was 9 months before Ben was born, at which point he as being conceived, hopefully involving my grandfather!