why would palp take on an apprentice who is older then him?

Why would he care? Dooku was a means to an end, like all Sith apprentices. Palpatine could care less if Dooku kicks the bucket from old age after learning the ways of the Dark Side as long as Dooku helped Palpatine achieve his goals. Though given Palpatine's clairvoyance, he probably knew Dooku wouldn't die of old age. Most Sith don't.