Speaking about Timing...
There is another aspect of timing activities for young children beyond when is the optimal age to start a new activity, and that is duration. How long should you give an activity before you make the decision to move on to something else? And for that matter, what is the criteria for sticking with something versus moving on? Enjoyment? Progress? Utility?
Let’s unpack each of these criteria and then return to the question of duration. One criteria for whether to start a given activity and whether to stick with it later is utility or how useful the skills gained are for “later life.” Many would argue that being able to read and having a basic mathematical understanding are very important, if not essential, skills for children to acquire. Swimming, at least to the point of not drowning if one were to accidentally fall into a pool or off a boat, could be considered a critical life skill. In comparison, being able to play tennis may not be as essential; although developing hand-eye coordination is useful. By the utility standard, perhaps one sticks with an activity longer because mastering it is more important. For example, maybe parents should force their kids to endure swim lessons until they are “water safe” regardless of whether it is enjoyable for the child or efficient progress is being made - which often go hand in hand.
Speaking of enjoyment, how much of a factor should that be in whether a child is encouraged (or forced) to continue an activity? The problem with this criteria is that we, as adults, know that learning something is not always enjoyable. There is inevitably a point of frustration that sucks all the fun out of something, even if it was incredibly fun when we first started. As adults, we also know that often if you push through that point of frustration, the reward is that the activity is more enjoyable than it even was initially. I think a classic example is learning how to play an instrument, particular the piano. The learning curve at the beginning is not too steep and most children can learn how to play simple melodies without too much difficulty, and often with quite a bit of enjoyment. But inevitably, the learning curve steepens and the effort to progress ratio increases dramatically, making practicing the piano frustrating, burdensome, and just plain unenjoyable for a time. Parents are generally not sadistic and wanting to torture their children but should a parent pull the “I know better than you do” card and force a child to continue piano (or baseball or computer programming or cooking or painting) when it is no longer enjoyable? It would of course be much easier to do that if there was a defined time period of the unenjoyment that was known upfront - e.g., stay strong and ignore your child’s tears and pouts whenever you make him practice the piano for 6 months and then it will be over and he will love playing again. But alas…
The final possible criteria to consider is the progress which is being made by the child (often relative to the effort on the part of the parent to drive the child to lessons or practices or to help the child practice). This criteria is also problematic in that progress rates are rarely if ever steady. One might see a lot of progress in the first 6 months and then very little for the next year and then steady progress thereafter. Many parents pull their children from activities when the progress starts to stall. After all, why should one keep investing time and effort into something with little to no return week upon week? However, how do we know if the lack of progress is a plateau (and after some additional time and effort we can get back to an upward progression) versus a wall (all the time and effort in the world is not going to get your child to be a gymnastics gold medalist).