Oldish means fairly old. The person or thing is not young, and you are not saying they are ancient either — you are staying in the middle. An oldish car still runs; an oldish building is not new; an oldish man is getting on. The ending -ish has the everyday meaning “somewhat, rather,” the same as in youngish. Casual, spoken more than written. It is about age, or how long something has been around. It does not mean “in the style of the past”; that is old-style or old-fashioned.