I would look at the area that is thinning. Can you see any new growth? If you can it means the hair is growing back. Have you had any chemical treatmets? Coloured you hair etc? If you have it could just be breakage from the chemicals and will prob grow back. Remeber, hair grows aprox 1/2 inch every 6 weeks so may take a while to show any progress. Hair also goes through thress stages, growing, resting & falling out. If your hair wasn't in the growing stage it may take a while for you to notice if it is growing back. The hair will not all be at the same stage though so you may have some 'sparser' new growth- don't panic.
If the hair is not groming then it could be male pattern baldness- which happens to most men at some point. Some ealier than others I'm afriad. Or alopecia. Alopecia can happen for a number of reasons, stress, illness, diet lacking in vital stuff. Go to your doctor and have a blood test done if you think it may be that the hair isn't growing back. They can rule out any medical reason.
What age were your grandfathers and father etc when/if they starte losing their hair? Male pattern baldness is generally genetic so if you look at that it can give you an idea of the age you may start to lose your hair.
Hope I have helped in some way and not depressed you too much.
I am a hairdresser not a trichologist and these things are meant to be dealt with by a trichologist so if you think you may be losing it and are worried it isn't your 'natural' time to thin, you should consult a medical professional.
Sorry bit of an essay.
Good Luck.