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Labour, the Lib Dems, the SNP and the Greens are all back votes at 16 - only UKIP and the Tories are against it.
Sixteen-and 17-year-olds were allowed to take part in 2014's Scottish independence referendum.
But Mrs May will tell BBC Radio 4's Westminster Hour she believed it was "right" to keep the current minimum age of 18 at UK-wide elections.
"This is one of those questions where you have to draw a line," she will say in an interview to be broadcast at 2200 GMT.
"You have to pick a point at which you think it is right for the voting age to be. I continue to think it is right for it to be 18."
How many people are registering to vote?
Would visiting Parliament inspire you to vote?
Greens demand votes for 16-year-olds
In an interview with presenter Carolyn Quinn, she argued there are other ways for young people to participate, saying: "The implication from your question is that the only way to get engaged in politics is by casting a vote.
"I think it is important young people watch politics, pay attention to politics, get to think about their own views and where possible start to get involved."
Asked what Conservatives were offering to appeal to young people, she called attention to forthcoming talks on Brexit.
"We have to get those negotiations right and we have to get them right for those young people's futures," she said.
She was also challenged on changes to the housing benefit element of Universal Credit which exclude 18- to 21-year-olds.
She told the Westminster Hour: "I don't think any of us wants to see any one sleeping rough on our streets.
"We are putting £500m pounds over these five years into homelessness, into preventing homelessness."