I've taken Propecia since I was 28, I didn't feel any side effects, would there be a possibility of sudden future side effects?

If you don't have side effects with a medication you have been taking for several years, it is unlikely you will have side effect in the future.  It may be possible but it is unlikely.  See your doctor if you are concerned. 

It would be highly unlikely to suddenly develop side effects from a medication that you have been using for years, however it is a possibility. As we age, our natural hormone levels have a tendency to diminish which could affect this particular medications potential effects.

In long term users of finasteride, the body has had a long time to adjust to the reduced Dihydrotestosterone (DHT) levels as well as the other hormonal changes (i.e. changes in testosterone and estrogen) that accompany finasteride use.Suddent side effects are unlikely but not impossible. Overall, however, side effects are still possible with finasteride. The incidence of new side effects in long term users is likely lower than men just starting the drug, but the risk is not zero. Men on finasteride, even long term users should be speak to their prescribing physician about side effects to watch out for