Botox treats motor wrinkles which are wrinkles that appear at the movement of the muscle and disappear at the end of this movement. When these wrinkles turn into persistent wrinkles on the skin, even without the movement of the responsible muscles, the use of Botox injections becomes partial, in the sense that it improves the wrinkles without completely removing them.
There is no specific age for injecting Botox, as long as it is used to treat motor wrinkles that may appear early or delay in appearance. Meaning that Botox can be injected based on skin condition and wrinkles and not depending on age.
I often hear this comment before Botox injections and the answer is the exact opposite. Wrinkles improve after Botox injections even after the effect has disappeared, because the skin has long been relaxed without any wrinkles, if it is treated and protected at the same time.