Hmmm. It's based on an average wage, presumably the mean wage of each age group. The problem with that is the mean average wage in the UK is rather higher than the typical wage.
Consider 100 people:
25 of them earn 12K/year
35 of them earn 18K/year
20 of them earn 25K/year
12 of them earn 30K/year
5 of them earn 50K/year
2 of them earn 75K/year
1 rich capitalist earns 500K/year
Assuming you start earning at 18, you'll need to average 27k/year to hit a million by 56. The average wage of the above group is 27k/year. How many will actually earn that million? Only one in five.
I don't know how well my hypothetical hundred people reflects the real distribution in the UK, but it explains why I don't trust 'average'.