I've had this guitar for at least 10 years. It was my first electric guitar. I have never taken it to get any adjustments over time or anything like that. For a while now I've noticed that its tone is off. On my high E string, the harmonic on the 12th fret is a lower tone than the actual note itself. How would I fix this?

....actually now that I check, every string is higher than its harmonic on the 12th fret

Oh and I have a Fender Strat