The best time to start is when you're keen. You give yourself double the chance to succeed. When you put it off, it becomes days then weeks and then eventually you find yourself years later stuck in the same position.
In 10 years as a trainer i've never seen anyone who kept putting it off succeed and get the body and life they wanted, yet ive seen hundreds of people who said "lets do this" and change their lives.
The hardest part is just starting. Anyone can do it, all fitness levels and ages, anywhere you like at anytime.