I would be careful to conclude causality here. There are a lot of confounding factors in these studies. For instance, most childless people are not childless by choice but due to already existing illnesses which predispose them to shorter lifespans. Socioeconomic factors also play a huge role. For instance, the numerous studies which show that adoptive parents live longer than their childless counterparts need to account for the wealth and lifestyle factors that allow these parents to adopt kids in the first place.

There have also been studies showing an inverse relationship. Nonetheless, a clear fact is this: "People with children live longer than people without". IMHO, they need a robust study on otherwise healthy and socioeconomically stable individuals (perhaps otherwise childless by choice sperm/egg donors) with healthy counterparts. In order to see, if something like, "In order to increase your longevity, its good to have kids." is reasonable or not. Personally, I'd say the judge is still out on that one.