The assessment for Gillick competence can`t be applied to voting in the same way.
I`ve highlighted a couple of stumbling blocks.
Source:
https://learning.nspcc.org.uk/child...lick-competence-fraser-guidelines#heading-top
Professionals need to consider several things when assessing a child's capacity to consent, including:
the child's age, maturity and mental capacity
their understanding of the issue and what it involves - including advantages, disadvantages and potential long-term impact
their understanding of the risks, implications and consequences that may arise from their decision
how well they understand any advice or information they have been given
their understanding of any alternative options, if available
their ability to explain a rationale around their reasoning and decision making.
Remember that consent is not valid if a young person is being pressured or influenced by someone else.
Professionals able to assess competence wouldn`t include the volunteers at a polling station.
Voting is politicians influencing someone almost by default.
Mind you there are plenty of people, over 18, who shouldn`t be allowed near a ballot paper...