You might mean:
if ( (car == "I" || car == "J") && agec < 25)
But you didn't tell us exactly what it's supposed to do!
yeah sorry, should have gone into more detail, this if for a online booking form,
var car is a letter which matches a car on my fleet, letters I and J have a minimum age limit on 25 and K K2 L and J2 of 28,.
var agec is linked to text box that once they have filled in there d.o.b it figuers out there age.
what i need this to do is only fire if there age is under either 25 or 28.
Looking with google chrome the script knows the age is over 28, but still fires the alert