12 years old.
There are no strength requirements for any of the intro classes or workshops or parties. Not all poses are acrobatic. Many poses are done standing on the ground, sitting or lying in the silk.
Wear comfortable, fitting clothing that covers your armpits and the backs of your knees. This is for hygienic reasons and also to protect your skin from potential friction from the silk. Your clothing should also be snug, so your shirt doesn’t fall over your face if upside down. You should also tie long hair back, remove any jewelry and have trim/filed nails.