There is no problem with a hyphen in a string like '25-34'.
Is your Age field a bucketed range already containing the literal strings '<25' , '25-34' , '35-50' ?
However if your Age field is a number indicating Age then you will have to do this:
If( Age <25 ,
If( Agr >= 25 AND Age <=34 ,
If( Age >= 35 and Age <= 50,