Recommended age for technique:
No precise age for discontinuance.

Description of technique:
Place a hot water bottle on baby's bed prior to rocking and night feeding (not too hot, of course!). This can prevent that Cold Sheets recoil "Waaaaaaa!" phenomenon (Mom and Dad's arms and lap are so much warmer than winter sheets). Flannel sheets feel good in winter, too. Remember to remove the bottle and put it out of baby's reach.

Other comments or suggestions:
Look for a small model of hot water bottle, vs. those oogly big red ones, or sew a simple flannel cover "bag" with sewn-in ties near the top. This would actually be a considerate baby gift.

My daughter and son both had colic (cranky belly ache, at any hour, 4 months plus) as infants. Sometimes I'd hold the flannel-covered water bottle right next to the little pajama-covered tummy, and there would be Peace in the house. (It would not always work like Magic, but when it did, it was sheer bliss for both of us.) It was far better than using something with an electric cord. Our hot water bottle is light blue, and has some silly design on it, like the 3 little pigs, but I still like to put a flannel cover on it. We still have it; heat therapy (and cold therapy) have applications in sports injuries...and for aging parental body parts. ;-)