Because he's been bogged down with injuries for the past two season? It's fair to say age has affected him, in that injuries have become a lot more common for him, but the right treatment and care can see him hit form again.
Scholes and Giggs are almost 40. Gerrard has just entered his 30s. It's ridiculous to start writing his eulogy right now.
You're right, he has been hampered by injuries but those injuries and more specifically the time and effort put in to recover from them, whilst frustrating at the time and limiting his appearances have also taken their toll long term in my view. In 08/09, supporting Torres with the brilliant partnership in Masch and Alonso behind him, he looked superb again but that was some time ago. Last season, I thought he might have just been suffering from the absence of Alonso but I'm not so sure now. I wonder if he'll ever be that same player again. I'm not writing his eulogy but I'd personally like a fit and firing revitalised Gerrard to be the icing on the cake, an unexpected bonus rather than something we look to build around and I'd be amazed if we got anything like a full top form season out of him again.
Also, Gerrard's ability is based in no small part around his supreme athleticism. Scholes with his, in this country at least, peerless technique was always going to be able to adapt as his body grew older. Giggs has been more of a surprise by adapting as well. Whether or not Gerrard can evolve his style into his 30s is still very much up for debate. Early signs of him playing a deeper role in central midfield have not been too promising in my view.
I'm not writing him off though. I still think we can get more good seasons out of him as the most advanced in a midfield 3, hopefully sharing the role with Aquilani.