Question

How should you write so that elderly people will understand you ?

Answer 1

Hi

You should write using  with a correct grammar , no slang, no urban words as shorthand forms as "idk, lol,..." or any text messaging, a correct spelling and it should be allright. 

hope this helps !

Answer 2

For elderly people to understand you, you have to write formally.

Do not use abbreviations of any sort. Be polite in your writing as well.