I'm no expert about this topic (effects of society on evolution), but it seems to me like after a certain age, there are diminishing returns from having grandparents. One critical point might be when you get a stable job. After that, the role of grandparents probably can't compete with other selective pressures to keep those grandparents cancer-free. Right?
As to having children later in life, that could certainly help the situation, but only if we all agree to do that. If there is some genetic factor that determines when a person wants to have kids, then people who want to have kids earlier will likely win out.
Also, keep in mind immortal humans may want to bear children at ages much later than current ones can. For instance, it would be helpful if you could bear children at 30 year intervals so you could be sure you raised your kids right (or try a different approach if you didn't)