Approximately 1.3 million people have turned 18 since the General Election in 2017.

That works out as roughly 2,000 new voters on average for each of the 650 constituencies in the UK. Many of them are students. Many have no fixed address. Many will find it difficult to register to vote. But these are all potential Jeremy Corbyn supporters and Labour voters!
According to the Office for National Statistics, the total number of 16-17 year olds is down slightly from a peak of nearly 1.6 million in 2008. The figures are approximate over the whole country and it is impossible to calculate how many in any one constituency.
What we do know is that 18-24 year olds were in general very opposed to Brexit and are the age bracket most enthusiastic about Jeremy Corbyn.