It’s wonderful to hear that you enjoy the gym! Although someone else may be able to give you statistics on seniors and exercise, I say the sky is the limit! If you are in good health and can do push-ups and planks, go for your own personal best (think about hiring a personal trainer if you need more guidance or a specific program). Every individual is different and can achieve great things regardless of age–I see it every day in the gym.
Good luck to you and keep moving. You will be an inspiration to others!