An oldster is an old person. The word is relaxed and a little old in style, and it was built to sit beside youngster. You might hear it in a story, or when someone is teasing in a warm way, more than in careful school writing. It does not by itself mean “wise,” and it does not mean “weak”; it only points to age. Though actually the pair is not even: people say youngster all the time, and oldster much less. Some people now prefer older person, because oldster can sound as if you are putting someone in a box labelled “old.”