It is always a good time to start a yoga practice, and the earlier you start in your adult life the better. According yoga and ayurveda the best time to start is when the seasons are changing into Spring or into Autumn. In Spring and Autumn the body is more receptive to change, and flexibility will progress easier and strength accumulated faster. From these two, Spring is the best time of the year to start with yoga, as Spring is the season of growth. In winter and summer the body is more steady and the yoga practice will be more stable.
Early morning around sun rise is the best time to begin your yoga practice (which can be any of the limbs ; asana, pranayama, meditation etc) as with the change from night into day the body is the most receptive for change. It is also the time that nature is calm, quiet and concentration comes easier and natural.
Children from age 7 or 8 can start with classical sun salutations. Around 17 or 18 an Ashthanga vinyasa practice could be started. From the age of 12 next to the sun salutations Hatha yoga asana can be introduced, which should be hold for short duration and practiced more dynamically and with repetition.
After the body turned 28, the regeneration process is slowing down and the body starts losing its youthfulness. Joints (connective tissue) can shrink and fuse. Healing an repair takes longer time and is executed with less quality. The body intelligence works according the “when you not use it you lose it”. For some time this process is reversible, though the older you get it will become harder and will more time to slow down, stop or to reverse this process. At all stages and ages yoga can definitely help.
Any kind of stretching exercises you have done in your past, down to childhood, the body will remember and it will be a little bit easier to get some of that openness back later. It will also go a little bit faster. Being exposed to stretching on an early age will definitely benefit you at an older age. Whenever you do yoga (asana, pranayama and meditation) it will be all stored in your “health ” bank account, to receive that profit later in life. Keep in mind that the later your started in life with yoga, it will take longer and more effort to regain your natural openness , but it won’t be impossible. And with all the benefits, directly, short term and long term, it is never too late to start yoga.