No, Jimmy Kimmel is not too old to raise a third child at the age of 46, because that is common anymore. In today's times, it is not uncommon for the mother to be that old. People are living longer and they are healthier into an older age. If he wants to raise a child, more power to him.
Leaving aside, Jimmy Kimmel for the moment, I'd like to point out that many grandparents are functioning as "parents" to their grandchildren. Jimmy Kimmel at 46, is not too old to raise a third child. Plus, he's quite wealthy, so he can afford to have a lot of help on hand, not just for the newborn, but for his other children. It's not like he has to personally deal with cleaning, shopping and laundry anyway.
I do not believe anyone is too old at 46 to raise a child. Jimmy Kimmel is a lively, energetic 46 year old who is more than capable of keeping up with and raising a new child. If he has made the decision to father a child, then we certainly need to respect his decision.
The problem with people talking about rich people "being too old" to have children is that they forget that they have an entirely different level of options than the average person. They don't necessarily have to choose between having kids and their work, hobbies, and retirement because they can hire a lot of help and still be loaded. If Jimmy Kimmel and his second wife want a child, that's fine because they can provide better for 10 kids than the average person can for one. The newest Kimmel is going to be okay.