Is it permissible to increase the frequency of health screenings for employees in line with age?

Yes, but only if the employer has a very good reason for doing so. Otherwise, such a practice will amount to direct age discrimination, as it is directly linked to age.

Employers should ask themselves why they want to do this. In order to justify such a practice objectively they would have to show that it was a proportionate means of achieving a legitimate aim. This is unlikely to be possible if the increase in frequency is simply based on an assumption that older employees are less healthy than younger employees and therefore need to undergo more frequent checks.