The GOP cannot raise the voting age to 21

 Republicans are on record claiming they want to raise the voting age to 21. This is due to the fact that voters in the younger generation have shown a surprising resilience to GOP propaganda, and the Republican Party is afraid that as the Baby Boomers and Gen-X-ers pass away, they will have less political control. Their concern is highlighted by the fact that some analysts claim that the more liberal tendencies of younger voters persist across all regions of the US, meaning that traditionally deep Red states could start turning purple as soon as the 2028 election cycle. 

The Republican Party does not have legal authority to raise the voting age to 21. The 26th Amendment to the Constitution reduced the voting age for State and Federal elections from 21 to 18. Repealing the 26th Amendment would require a Constitutional Amendment, just like when the 21st Amendment was passed to repeal the 18th Amendment, the process which ended prohibition. Passage of a new Constitutional Amendment requires a two-thirds majority in both houses of Congress, and ratification by three-quarters of the States. 

Some Republicans will try to argue that this goal could be achieved through a Constitutional Convention, but that hope was effectively destroyed during the 2022 mid-terms, when Democrats were able to enjoy substantial gains in State Legislatures. A Constitutional Convention requires the agreement of 35 states, and given the political leanings of younger voters, that condition will not happen at any point in the near future. That goal is made even more difficult for Republicans due to the fact that information warfare analysts are getting better at describing how foreign information warfare agents work "both sides against the middle" by having pseudo-liberal propaganda agents encourage progressives to call for a convention, a process that any rational person understands would be dominated by representatives of dark money interests.