I'd say early teens. Puberty, hormones, dealing with superficial peers, wanting to be independent and have the rights that adults have but being too young to have that.
Next worst would be infant or very old age if you live that long. Infants are basically helpless on their own, can't get around and explore yet and can't adequately communicate your needs outside from crying.
Very old age can be kind of similar to an infant in a way. Physically, you might not be able to get around well. Mentally you might not be functioning well anymore.