This is one of the questions I'm often asked so here are my thoughts on this.
If we are just talking about principles and applying them at home, I don't think there is an age limit. Your family will benefit from embracing Montessori Principles at home and you don't need any special tools to start, just use what you have at home. You can apply as little or as much Montessori as you want.
For example is the principle of FOLLOWING THE CHILD, meaning following their interests and pace in learning. People learn better when they are interested in what they are learning. Do you agree?
Through observation, we can see what a child is READY to do and what his/her interest is. Try introducing just 2 toys and see which one your child gravitates and engages with. They don't need a lot and by that I mean a 3 year old can just have 6-9 options per toy rotation (as for a baby below 1 year old - way less).
When applying the same principle to a high school student deciding on a career path, it's as simple as respecting their choices and not imposing what we want. I saw Jokoy's stand-up and one of his jokes was how Filipino moms tend to dictate what career path a child should get. How about we let them discover what they're good at and what will make them thrive?
However, if we are talking when is the best time to send a child to a Legit Montessori school, bear in mind that it usually starts at 14 months old (for Infant Community). The earlier a child is introduced to a prepared environment, the better a child can maximize his/her potential (I'm basing this answer on what I read in books and discussed with some educators here and in US).
In the Philippines, there are some legit Montessori schools that start at this level. Majority start at the Primary level though (3-6 years old).
Dr. Montessori did say: "In the first three years of life, the foundations of physical and also of psychic health are laid. In these years, the child not only increases in size but passes through great transformations. This is the age in which language and movement develop. The child must be safeguarded in order that these activities may develop freely."
Hope you enjoyed reading this :)