[amaccuish]

I see it more as several stages of life, some easier than others:

baby/toddler growing up in a multilingual environment = fairly easy

baby/toddler growing up in a monolingual environment = difficult

Young child -> teenager (e.g. I actually began learning french in primary school) = slow and difficult

Late teens -> young adult = the best time after baby/toddler

+40 = more difficult but not impossible