Source: http://www.who.int/growthref/en/

At least, when set to use CDC data.

That is, if a preference for CDC data isn't indicated (which also
covers that range).

That is, an age of 60.1 months should resolve to a table for 0-5 year
olds, not a 5-x table.

Source: http://www.who.int/growthref/who2007_weight_for_age/en/

(In the process, simplify (for readability) the logic for resolving the
WFA tables.)

Sample usage:
    calc = pygrowup.Calculator()
    calc.acfa(measurement=12.5,
              age_in_months=47,
              sex="F",
              )
Alternatively, there's an alias:
    calc.muacfa(measurement=12.5,
                age_in_months=47,
                sex="F",
                )

Source: http://www.who.int/childgrowth/standards/ac_for_age/en/