What’s an alternative to placeholder text in forms?

I'm designing a form for elderly (65+) and I'm focussing on good readability and high contrast.

I want to let them know which format they have to use while typing their birthdate (e.g. dd-mm-yyyy), phonenumber etc. I can't use placeholder text because of it being less recognisable as an input field, bad contrast (when I use grey text) and a higher chance for errors.

I did some research and an option, although debatable, would be floating text labels. The reason why I can't use this is because the font-size is too small and it wouldn't really solve the problem because otherwise I could just put the required format in the field label.

What is an alternative I can use?