I feel that it depends on the potential "mom". Everyone's so different.
My cousin just had her 3rd at 40 and she's still so energetic and caring that I would be happy to see her have more. I look up to her for wanting a family so bad and going for it...they didn't have children until after she was 35.
Another of my friends had her at 45. She said it really "younged" her up. She was the cool one of her age when her daughter was in high school. All of her other friends were out playing BINGO or were home alone and she was at football games, teen outings, etc. It worked well for their family and she stayed very active in a much older age than others did.