This question was raised up by my sister who was with me, and we were walking around in Ngee Ann City and we saw a girl, looks about 5 years old, who wanted her dad to pick her up. So she kept raising both hands in sign for him to pick her up, for a couple of times, and finally he did and she held her hands around his neck and was laying her face on his back/neck contently.
My answer is this:
For as long as I physically can; I would love to carry all my children for as long as I can.
It's a joy to carry them, and for them to want to be carried, still. I love every moment, the harsh lack of sleep, to the twinkle in their eyes and their smiles and cries.
I. Love. It.