1. Younger ones are being hired, older ones are not given the chance
This is quite a clear indication of age discrimination, especially if the company/firm has a repetetive history of doing so.
2. You start hearing insinuating comments about your age.
You may receive these both by colleagues and your boss. Comments like: ”Somebody younger may do it better/faster/more efficient“, "Say, when are you going to retire?" — clearly show what one may think about you and your position in the company. Make sure you prepare a good response and let them know that you have no plans to retire and that you intend to work there for a long time. Do it in spoken or written word, but remind your boss that you have no such plans in near future.
3. You are constantly assigned to unpleasant duties.
Job reassignment can be the clearest sign that they're trying to replace you or get you to quit. How to avoid this if you really plan to continue working in the same company/firm? Work on yourself continually and do not miss chances to improve.
4. You stop getting raises.
If your colleagues are getting raises for the excellent performance they give, that is OK. But if you are also giving excellent performance, and aren’t getting any raise – this may be a sign of age discrimination — unless you're already at the top of the pay scale.