You could probably find a discount, though I doubt it would be based on the age of the new member. Regardless, remember that the younger the Life member, the more years of benefit they will receive, effectively lowering the price per year.
Congrats Gramps... Relish the moments, It won't be long and you'll be taking him to Baseball practice or to DC for SAM Marches... Relish the Moments they go past so fast...