adj. 25B6; adjective
irresponsible behaviour : RECKLESS, rash, careless, thoughtless, incautious, unwise, imprudent, ill-advised, injudicious, misguided, unheeding, hasty, overhasty, precipitate, precipitous, foolhardy, impetuous, impulsive, devil-may-care, hot-headed, delinquent; N. Amer. derelict.
an irresponsible teenager : IMMATURE, naive, foolish, hare-brained; unreliable, undependable, untrustworthy, flighty, giddy, scatterbrained, harum-scarum.
sensible.