Income is only weakly indicative of class, and becomes less and less correlated over time. There's a very shoddy understanding of what class actually means in modern journalism, I think because there's not enough study of the history and historiography of economics required in most undergraduate economics courses that then go on to inform journalists. Your class is not your income. It is not your wealth (although it does track wealth much better than income).
Your class is your relation to the means of production, or put another way, the extent to which you rely on labour rather than on capital for your means of subsistence, and then the type of labour and the type of capital you rely on.
The majority of capital-ownership in this country is in property, so the single biggest determinant of class is whether you own your own home compared to whether you rent.
The second biggest determinant of class is your expectations of what your capital relationship will be - whether you're trying to run up savings or in the process of using your savings, or in other words whether you're retired or not. In the last approximately 30 years the retired have emerged as a distinct class because of the gap between retirement age and life expectancy (and this is why we see such a large divergence in vote with age that didn't exist 30 years ago).
The third biggest determinant of class is the rate at which we expect this direction to move, and this mostly applies to those accruing capital. We're not concerned with present earnings here but expected earnings - e.g., 4 years ago I earnt nothing (being a student) but putting me in the lowest class would have been an exceptionally stupid thing to do since my expected lifetime earnings are quite high thanks to my degree and education. Happily, education is one of the best predictors of lifetime expected earnings.
Now try plotting Labour and Conservative voting shares against: home-ownership, employment status, and educational attainment. Noticing a big correlation? Bam, class matters. Age matters because it tracks these.