I think attitudes to teenage parenthood are quite complex and may depend on where you live.
Where I am becoming a young mum is the norm really - education is poor, employment opportunities are limited and for many 15 or 16 year olds having a baby is no big deal. They have no real aspirations beyond being a mum, have no expectations beyond the immediate and cope pretty well with being young mums.
The benefit system means that you can not work, be a parent and not struggle unduly (not benefit-bashing at all, but it's very different to how it was in the 70s or 80s for example).
Interestingly, where I work (health centre in a deprived area) we see very little PND in young mums, far more in mums in their 30s. Young mums tend to have low expectations of parenthood (ie they're quite open minded and know what to expect having young siblings and larger than average families), they have lots of familial support often still living at home, no career to put on hold, lose or stall, and don't suffer financial loss as they had none to start with.
So I don't really understand why we are so anti- young parents. Most cope pretty well and are raising their children well.
I imagine the negativity is due to a perception that they are slappers, scroungers or wasters, contributing nothing to society but taking plenty. For some that may be true but for many it is far removed from the truth.