hyphenating from-to-brackets for age

[politely_curious]

Hi everyone,
I am currently abroad with no access to my Michael Swan and I'd like to make sure I'm hyphenating the following phrase correctly: 18- to 25-year-olds. This spelling is my best personal guess, but I would like to double check if the first hyphen (after 18) is a) correct and b) mandatory? 
Thank you so much!