If Tom Brady has proven one thing over the past few years, it's that he knows how to fend off Father Time. At an age when most NFL players are long retired, Brady only seems to be getting better. 

Despite the fact that Brady has been playing at a high level this year, at least one former NFL quarterback says he can already see a decline in Brady, and it's someone who would know exactly what to look for: Warren Moon. 

Not only is Moon a Pro Football Hall of Famer, but he played in the NFL until age 44. As a matter of fact, Moon is one of just three players in NFL history to throw for more than 3,500 yards and 20 touchdown passes in a season after turning 40. The short list includes Moon, Brady and Brett Favre. 

With Brady now 41, Moon says an eventual decline is unavoidable. 

"Age is something none of us can avoid. Things are going to start to slow down," Moon told Bleacher Report in a recent interview. "How fast? We really don't know. You can start to see some decline in Tom."

As someone who went through an age-related decline, Moon believes the first ability that Brady will lose are his reflexes. 

"The numbers are starting to show that there's a deterioration coming," Moon said. "Especially when you talk about the blitzing because that's something he used to really excel at. You do start to lose some of your reflexes and responses. I started to figure that out later on in my career, too. Even though your mind knows where you should be going, sometimes your body just doesn't react as fast. You're starting to see some of that with him."

Of course, the good news for Patriots fans is that Moon doesn't think Brady is suddenly going to hit rock-bottom. Due to Brady's intense offseason training, Moon thinks the decline will be gradual for the Patriots quarterback, who will turn 42 before the start of the 2019 season. 

"I don't think you fall off a cliff, but it is gradual, and once that gradual [decline] starts, there's no turning back from it," Moon said. "It's not like you're going to recapture what you had before."

Even if the decline doesn't happen quickly, Moon doesn't see any situation where Brady is going to actually improve his game if he decides to keep playing over the next two or three seasons. 

"I don't think he'll be able to go back and have another offseason where he can get back to the form he was two years ago," Moon said. "It's going to continue to keep sliding, and you just have to continue adjusting your game, and hopefully your offense and your offensive coaches will adjust and not have to rely on you as much to carry the football team as you have in the past."

Brady might not be having the best season of his career, but he's still been impressive in 2018. The Patriots quarterback is on pace for 4,554 passing yards and 28.3 touchdown passes, which is just slightly behind the numbers he put during his MVP season in 2017 (4,577 passing yards, 32 touchdowns).