Possible Issues With Water Heater

I have an old gas water heater. Roughly 15 years old (2010 MFG date) it has went strong until about a couple of weeks ago. Before it would just take longer to reheat but now it is not getting as hot. I havent tried draining it just yet, but I did turn it off and reignite the pilot light to see if there was an issue there. When it was relit, the flame did not expand to heat the water until I had it on the hottest setting. Even after waiting a few seconds on the others. Will draining possibly help this issue? Or should I get another unit? Thanks in advance