Are they trying to scare off what remaining youth voters they have left?
Absolute rubbish.
They also haven't thought it through, someone turning 18 at the beginning of September (say the 3rd) won't have finished their A levels and so would have to miss a year whilst they did their national service before then doing their second year the year after, although it gets worse if someone is born part way through the year (say) March as they'll be able to do most of their studies and then not be able to do their exams until afterwards.
For those with birthdays after the exams, where do they do their volunteering? Near their home, or near where they are at Uni?
It appears that they can only volunteer for the NHS, Police or Fire. Which sounds very much free labour to save having to pay for people to do those tasks.
Therefore, the young will have to work for free (25 hours when their 18), not be able to afford a house, two of them pay for one retired person (rather than 2.7 currently) and probably not be able to retire as then there'll only be 1.7 people (or less) supporting them in their retirement.
It'll also cost the government £2.5bn a year.
"National Service" is a phrase which will go down well with some, however most likely not all that many who would like the idea would have actually done National Service in the UK.
Whilst it could be argued that it's a good idea for those who need structure, are heading down the wrong track, etc. then it could be offered to them without offering it to everyone (and cost much less).