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

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