I'm young (19) .... and I am a mummy. But, because of the attatched stigma I guess, I do not ever refer to myself as a 'young mum', and I don't think anyone really considers me as such. I don't live the 'typical' lifestyle of a young mother: I'm married, have a stable income and home... Life is pretty easy for us - I guess I'm really lucky. I'm not saying that young mothers who are single, or less well off financially, do a lesser job as parents - but I am saying that few people I meet think of me as a 'young mum' because they tend to expect younger mothers to be single and hard up - which is a stupid preconception tbh but I caught up with an old teacher of mine recently (who knew my age but nothing else...) and she asked me "How are you coping? It must be hard being a single mum at you're age - Does the dad take any interest?" Oh how I loved that smug feeling as I told her that I am married to him - should have seen her face! lol
To answer the question anyway, I'd say that what is socially considered a young mum in the UK is about 23 and under... But how good a parent is has very little to do with their age...