The full rules are set out on the DWP site at
http://www.dwp.gov.uk/lifeevent/penret/pr_pensions.asp . This is my interpretation based upon my own circumstances, so there maybe subject o bad guessing by me and also not relevant to you.
Currently for men( I’m not sure if it is the same for women) the NI pension is based on a lot of factors and you would be best applying for a forecast on a form BR19. which will give you your pension at retirement based upon your contributions to date. once you have that you can then decide which is best for you.
If you continue to pay contributions up to the Maximum which I believe is 44 years ( soon to be reduced to 30Yrs) over your estimated working life which is 49Years you willget your full entitlement. which means if you have paid all your working life from 16 , you will not need to pay after age sixty. (for example in my case I am being credited the years from 60 to 65??)
The government recognise that the cost of pensions is gonna be higher in the future, Viz we are living longer and paying less directly ( IE 30Yrs), however the age from which you can draw your pension is going up to 68 for both men and women (Ok girls now you got equality!! but its gonna cost ya). The information is mostly contained here with the logic and costing studies
http://www.dwp.gov.uk/pensionsreform/