The electoral register for each year starts on 16th February and runs to 15th February. During the year in which you become 18, your birthday should be listed beside your name so that they can know whether to let you vote (or not). The register is based on forms which are sent out in September/ Octoberish to each household, so it's theoretically possible for someone's name (and Date of birth) to be put on the form when they are only 16 (let alone 17). When the forms are sent out, it sometimes happen that the head of the household (i.e. the parents) do not read the form properly and do not enter the names and/or dates of those who are aged 16 or 17 even if they should. In my electioneering, I have come across a few cases of people who are listed in the year when they became 18, but without the date given, or not listed at all.