YouGov's MRP model is seen as one of the most reliable pre-election predictors of what results will look like in the UK. Their turnout predictions by age are as follows:
According to the academic analysis linked on their election centre, this is determined from historical figures, primarily in 2015 and 2017.
"In contrast to traditional polling approaches, we do not rely on respondents’ self-reported likelihood of voting when working out probable voting patterns across different constituencies. Instead, we build a model based on data from past elections to predict whether different types of individual would be likely to vote in a new election. The result of this process is that we assume that the electorate for a new election would be demographically similar to the electorates of 2015 and 2017 (which were themselves similar, despite 2017 claims about a “youthquake”). This is the same strategy we used for the 2017 election model. It is not perfect, as it is possible that there will be changes to the demographic make-up of the electorate, but is also unlikely to yield large errors overall."
It seems to me like these turnout figures exaggerate the differences that exist between age groups - compare them to the historical figures from the parliament website:
While youth voter turnout looks pretty much in line with what you'd expect (assuming that 2017 was largely an anomaly) their estimates for older voters seem significantly higher than the historical data would imply. Has anything been published which would explain this difference?