If you're asking about when would be a good age for a child to have fun acting and being in plays and the like - any age is fine. Taking acting classes, dance lessons, voice lessons can be fun for children who are interested in those areas. Being involved in school plays, community theater, church shows and the like can be a good creative outlet, a nice way to make friends, an activity to help a child learn life skills like resilience and team work, and just have fun. Joining a drama club and competing in forensic speech/drama contest is another good activity for children interested in acting. See what opportunities are available in your area.
If you're asking about professional acting - no age is the "best" age for a child to attempt a professional acting career. It depends much more on the child's maturity, temperament and the support of the family rather then a numerical age. Professional acting is a business. People are in it to make money not make your child's dream come true or help them become a mature, functioning adult.
It also depends more on the parent having a realistic understanding of what is involved in attempting a professional acting career then the age of the child. The parent needs to make sure that the mental and emotional needs of their child are met along with being the CEO of a business where your child is being marketed and sold. That is VERY hard to do (which is why too many child actors end up confused and in trouble as adults).
The parent needs to understand the business end of the industry - the casting process (agents, breakdowns, casting directors); the legal issues (contracts, unions, entertainment work permits, Coogan trust accounts, taxes); marketing (head shots, resumes, show reel, website, social media) and networking. They need to e able to make good decisions for their child and the child's career.
Most teen roles are played by adults because there are legal restrictions and requirements for minors in the industry. They require entertainment work permits, trust accounts established in their name, and tutoring/classes to meet educational requirements. And children are legally limited iin when and how long they can work. Adults don't have those restrictions. And frankly, physically teens can change a lot over a short period of time which can mess with filming and the like. Adults aren't going to go through some growth spurt or sudden breakout.
Plus adult actors usually have a more professional approach to acting then most teens and should be more emotionally capable of dealing with the responsibility of working a professional job. Not that it's impossible for a child and their family to handle a professional acting career - but it's rare for a child to have the ability to effectivley deal with the stresses of a professional acting career.
A lot of kids and parents have fantasies about a child being "discovered" and given an acting career - without a good grasp of the realities of attempting a professional acting career. So the numerical age of child matters far less than the parent's ability to oversee to the child's welfare when attempting a professional acting career.