The state, process, or quality of growing up; the condition of maturation and development into adulthood.
From growing up (phrasal verb of grow) plus the nominalizing suffix -ness, creating a noun meaning 'the condition of growing up.' This is a relatively modern or informal coinage.
This word probably appears most in young adult literature and psychology—turning the experience 'growing up' into an abstract noun 'growingupness' lets writers discuss it as a shared human condition rather than individual experience.
Complete word intelligence in one call. Free tier — 50 lookups/day.