It is amazing how even 10 minutes of exercise can change everything for anyone. Young or old, athletic or otherwise.
For example, I stay active but got out of calisthenics for other activities and workouts. For fun, at work we decided to see if we could do 100 pushups a day for a month. That is a little crazy we found as your chest and shoulders don't have the chance to recover but it started a new habit of 30 squats and 20 pushups every day before coffee for me. Almost negligible amount of time spent and even getting the blood flowing in the morning makes a huge difference.
Everyone can make a huge difference in 100 days!
If a person does something he wants, let him say that only I will do this, I will succeed in this, he cannot stand in front of any obstacle. Wonders can be created in 100 days, do sports, stay healthy.