Well,your kid is too small for school,but if the montessori has a daycare attached then they will take any kids from 3 months onwards,but it will be daycare,not school.Montessori preschooling starts only at the age of 2 1/2 to 3 1/2 yrs.
My son went to a montessori in CA for Pre-K at the age of 3 1/2 + yrs(he was almost 4 then).I don't know about your area but we had no problem in getting him an admission and we got it immediately.I don't believe that to get admitted into a montessori schooling system,you have to put your kid so early in that school and that you have to start with daycare to get admission to the school.
Another thing to consider is that a montessori form of education is good but any montessori school has only till kindergarten.After that,anyway,we have to find another school.I will tell my experience.Now my son is going to kindergarten here in a private school.Even though he went to a montessori for pre-k,here the school authorities don't give much thought to that.In his class now,half the students do first grade math and the rest of them do kindergarten math.My son falls in the kindergarten math group.I know for sure that he is very strong in math,but the school says they assessed him and put him in the latter group.Then I found out that all those who are doing first grade math are actually students who did Pre-k in this school itself! So,there is some kind of partiality here,don't you think.
As to your question regarding when they start benefitting ,surely they will start benefitting from it only after they attend proper school,that is around the age of 3 yrs.