Yes, generally there is no ideal age to start using preventive Botox, and it is considered safe for most of the adults when performed by a qualified dermatologist.
The decision to start using preventive Botox depends upon one’s own skin concerns. There are several factors involved in it as well, which is genetical, sun exposure, habits, etc.
While some individuals might benefit from it, others might not.
There are several temporary side effects associated with it too, which are swelling, bruising, redness, or tenderness at the injection site.

This is why a professional recommendation and consultation are highly suggested before
undergoing any treatment. Ultimately, preventive Botox can be safe in your 20s, but it should be done by a trained and well-informed professional dermatologist.