Ah! Old age.
If the answer to this question was the acquisition of material gains and wealth, perhaps almost everyone would be more serious with being very wealthy. But having wealth and being all alone is really really boring!
And if the answer had been making and growing babies, everyone would be serious with getting married. But, children will leave one day to have their own lives. Even though they may come sometimes to visit.
Maybe the answer in reality is each and every tangible things accomplished prior to old age itself. These are the things that will not make old age a regret.
Making good friends, raising awesome kids, gathering and acquiring some material things and wealth and assets, with utmost sincerity in all, might be the solution to relishing, and having a splendid old age,
if it ever comes!