No I don't think theirs was planned, and I don't think it matters. Their baby is obviously very much loved. He will probably be well looked after. He has a loving grandparent too.
I don't like how it's ok to be so prejustice against young parents. Give them a chance to get it right at least.
"you're in your 30s and have been married for 10 years and are emotionally and financially ready to raise a family and can't seem to get a bun in your perfectly well-functioning oven no matter how hard you try, seeing teenagers getting thoughtlessly up the duff is very galling"
It's is understandable that you feel that way but it is wrong to begrudge someone else something just because you can't have it too.
There are plenty of crap parents in their 30's and plenty of great parents in their teens, and vice versa.
FWIW, I had DS when I was 17. I was(and still am) in a long term, loving relationship with his dad, I was emotionally ready and I am a good parent, and we financially support ourselves. You don't need to be in your 30's to be any of the things you just listed.