How old should a child be before they start playing tennis? That may depend on how you define tennis. A very young child will gain no advantage or have any fun trying to play on a full size court with a regular tennis racquet and regular tennis balls. In fact it may turn them off the game all together and they will never return to play it when they get older.
That doesn’t mean however that you can’t introduce them to the skills of the game in a much more fun and scaled down version. This would be appropriate for ages 3-7 years.
Racquets of all types are available in lighter, smaller versions and there are balls that look and feel like a regular tennis ball but have a much more deadened bounce. Alternatively a nerf ball could be used.
Both of these types of products are a good start to introducing a child to basic coordination and timing types of exercises. You don’t need a court or tennis net but if it’s convenient all these exercises can be done on a court.
When the child can do all the above exercises they are ready to alternative the racket from the palm up grip to the back of hand up grip. Let the ball bounce to the ground between alternating each forehand and backhand version of this drill.
And for the super advanced version do all the above exercises with a real tennis ball and an appropriate sized and weighted racquet.