When should someone start learning sidespin (AKA english)?

One should use sidespin whenever it is required (see examples) or whenever it provides an advantage, but not if you are very likely to miss the shot (due to lack of practice with how to aim when using sidespin over a wide range of shots).

A person should not spend much time with sidespin until his or her fundamentals and stroke are solid. Also, CB control with stun, follow, and draw should be mastered first.

When deciding to use sidespin seriously, a person should also start learning about all of the effects (squirt, swerve, and throw) that need to be taken into consideration. Otherwise, they will miss many shots when using sidespin which can make them lose confidence, and their learning process (through trial and error) will be much slower. For more info, see aim compensation for squirt, swerve, and throw. A sidespin shot is much more difficult to aim due to how net CB deflection varies so much with shot speed, shot distance, and the amount and type of spin.

For more information concerning when sidespin should and should not be used, see the Sidespin Tutorial and the Sidespin Examples resource pages. See also:

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from Bob_Jewett:

As far as the basic knowledge part, I think it is important to discuss that before the beginner starts using side spin. I point out how the side spin can be useful but at the same time I show five major problems with using it: squirt, swerve, throw, miscues and cling/skid/kick/bad contacts, and some of the details of those problems. Most beginners do not fully understand those problems when first introduced to them, but I want them to be aware that problems exist.