For wrinkles. Botox is most effective at preventing worsening of certain lines of expression especially the glabella (between the eyebrows), forehead, and crow's feet , and perioral (smoker) lines. These can start forming as early as 20's and get more noticeable in 30s, 40s. Botox can be started at any age when wrinkles form but early intervention helps to keep the lines from deepening and worsening.
Botox. If you mean what age, then early 30's and many even believe late 20's.
When you feel ready. Some patients start as early as in their 20's if they're seeing prominent lines. They do it in preventive mode to reduce deeper lines from settling in over time...Especially if they see a family trend. Botox helps to relax the muscles that cause the expressions that create the lines. In this way, it can help to not only smooth the lines but also to change the habit of making those movements.
Varies. In my practice I have 25-80 year olds using it. When you start, depends on you. Its definitely worth a try. Use less the first time just so you can see what it does. The goal with Botox is to look natural, well rested...Not frozen like many of our actors. Speak with your cosmetic physician about your budget as well as your goals to help best address everything.
When needed. When the wrinkles start bothering you.
Up to you. Botox is a wonderful treatment for facial wrinkles among other things. But it is really up to you how much those wrinkles bother you. Many people opt to start Botox early before permanent creases may develop, others prefer to wait until things really become bothersome. Think about it well and discuss it with your plastic surgeon. Be well.