Agreed again .... we do not all fit in the same ole box of when we master things by the box therefore is just a general 'guideline' as with any 'skill based' milestone how 'quickly' it is mastered will depend on the personality of the person, the learning style, expectations being consistent between environments and the opportunity to practice.
For example while MOST 4 year olds who've been in childcare centre are able to cut with scissors along a straight line at least according to the 'developmental checklist' .... a child from a 3rd world country who moves to Canada at age 5 and has never SEEN scissors let alone had years of practice with them is going to pick them up and handle them the same way a first time scissor user at the age of 18 months or 2 years or whatever age you are comfortable to offer will ~ ineptly