As a general rule in the 1901 census you will see ages 'rounded up' to the nearest ten so there are a disproportionate number of people aged 40, 50, 60 etc, probably because people just did not know their age.
However, in 1911 Old Age Pensions were introduced for those aged 70 and over and as a result this encouraged people to re-consider their age and this explains a lot of the discrepancies between ages in the two censuses.