98-year-old celebrated his birthday on Dec. 13, and he was spotted working out at the gym just one day prior.In photos obtained exclusively by The Post, the “Mary Poppins” star was seen on an exercise outing with his wife, Arlene Silver, 52, in Los Angeles on Tuesday.The legendary actor was happy to chat with onlookers and seemed to be in a cheery mood as he headed into the gymnasium.