Define "Old age"....
Well Francis II will be the only king from his generation, his brother Henry stays in Poland, his brother Charles stays just a royal duke. He will probably have issue with Mary, which means a son will get France and Scotland. Elizabeth will probably do her best to stay on good relations with them especially as she herself won't marry. Elisabeth probably gets to have sons with Philip II since she doesn't die of childbirth here (old age implies that). Claude could possibly have even MORE children, not that she didn't have a lot IOTL. Hercules of Anjou spends his life as the king's younger brother.
Let's say at least until their fifties. Older if you want.
I agree that one of the brothers is likely to become king of Poland but not necessarily Henry as he would have two available older brothers (Louis and Charles) who may be chosen instead of him.
What about Netherlands? Assuming one of the brothers is still elected (as Alençon was in OTL), does he have a chance to keep it?
Also, there were, in OTL, other plans that came to nothing:
-Louis becoming duke of Urbino,
-Charles being elected emperor of the Holy Roman Empire after Maximilian II,
-Henry marrying Juana of Austria (Sebastian I's mother) who may receive Milan as a dowry,
-turning Corsica into an independant kingdom with Henry as a king,
-having either Henry either Alençon becoming king consort of England by marrying Elizabeth I.
How likely is it to have some of these plans achieved?
What about the sisters?
I guess Elisabeth and Claude would still marry Philip II and the duke of Lorraine. You're right: Elisabeth is likely to have a son, here. Said son would become king of Spain. Would he be a better king than OTL Philip III?
Would Marguerite still marry Henry of Navarre? I don't know.
What about the twins, Victoire and Jeanne?