According to The Wonder Weeks when they are going through their 4th mental leap, between 14-19 weeks, needing more head support again is one of the signs it's happening. It's to be expected and totally normal!:)
It all depends on a lot of factors. My children both had decent head control from birth. An adopted daughter in our extended family had a weak neck for months.
My boy is 21 weeks today. He has been holding his head up and looking around since birth, a little wobbly for the first few weeks but now he holds is head up like a champ. He loves tummy time because he does the cobra position and looks all around. He especially loves to get entertained by our 3 y/o boxer.
Re: Neck strength