These are facts:
The Federal government website that gives out voter registration information for the various states has been changed, sometime since February 22nd, 2018 (the last archive available on the wayback machine.)
The changes to several state listings are FALSE. Instead of giving accurate registration ages, they now contain the language "You must be 18 to register and 18 on or before the election to vote."
It is perfectly legal in most states, including the ones erroneously listed otherwise, to register to vote at 17 as long as you will be 18 on election day. Here's a specific example using Pennsylvania. Old site on the top left, new FALSE info top right. PA site on bottom.
Here is a link to the archive of the Feb 22, 2018 iteration:
https://web.archive.org/web/20180222154938/https://www.usa.gov/voter-registration-age-requirements
And the current version:
Compare for yourself if you don't believe me.
https://www.usa.gov/voter-registration-age-requirements
Now the important question...
Why would the federal government wish to give out false information which would discourage young people from registering to vote?