Especially a highly successful one. Besides his age--which is not an objective issue--what do Republicans have to run against? His son. That's the list. But both will be blunted somewhat by the time the election rolls around, and his opponent's age (and kids) will blunt it even further. But his age WILL matter because we're not a society that respects age and wisdom. So, many people will want to vote against him simply because he's old, not because of anything his age is doing to him. (Although his opponents like to make things up about it.) Even poor first-termers seldom see serious primary challenges. The last one was Ted Kennedy in 1980 vs Carter, but that really didn't do much except weaken an incumbent who was going to lose anyway.