"It is an specialised form of income tax where income under a certain age is taxed, but the same level of income over that is is not. It has a deliberate bias, therefore, that taxes younger age groups more heavily than older ones."

Only partly true. Or rather, it appears to contain a conflation of income-level and age. NICs are payable by everyone in employment between the age of 16 and state pension age. The rate is 0% on earnings below £797/month (£184/week), then a varying rate, depending on the individuals’ NI Category, of between 2% and 12% on earnings between £797.01/month (£184.01/week) and £4.189/month (£967/week). Earnings above £4,189/month (£967/week) attract a flat rate of 2% with no upper limit.

My take on that is not so much that it hits younger age groups more heavily than older ones (except insofar as it’s usual for younger people’s pay to be lower generally than older employed people), but more that it’s regressive in that, over the £4,197/£967 upper earnings level, the rate overall is likely to be lower than on earnings below that level.

In other words, it’s not loaded specifically against younger people, but against poorer people - it protects the well-off, exactly what you’d expect of Tory tax policy.