Before talking about perfect age for workout, at first we need to focus on what kind of workout we are preparing to do. Like push up, stretching, running are the common workout which does not have any age limit. We can start it at the age of 13-14 if you are involved in any athletics or other activities. There are many myths about height relating to workout like if you start joining gym in small age, your height stops increasing, it will give a problem in older age and many others but the fact is that, joining gym does not effect on your height. Average height of Nepal of male is 5ft5inch and for female is  4ft 11inch. If your height is above average then there is nothing to worry about age, as you can join at the age of 16yr, 17yrs and so on. Even Arnold Schwarzenegger have started workout at the age of 15 and was Mr.Olympia at teen age. The thing is that if your height is below  average, and your age is almost above 18yr+ you can start workout. Sometime what happens is that, height depends upon genetic as well.  If your all of family is below average than your height might stop below average  so there is nothing to be worried you can easily start workout. So, in the conclusion you can start workout at any age, it does not effect any thing,  but if you have still chance to increase height (Height increases till 30yrs) then you can just skip some of the exercises in your workout like dead-lift, and the exercises that you do above your soldiers.