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).