Matt Barrows recently published an article with a few quotations from Justin Smith about his uncertainty heading into the offseason. Since he is a free agent and 35, the specter of retirement looms large over the next year. He could decide, as Barrows puts it befitting Smith's Cowboy nickname, "to ride quietly into the sunset."
The motivation would be a desire not to be that veteran who continues to play, but with diminishing skills. That said, he hasn't "given any thought" to what he will do this offseason. Even if that's not true (I've got to imagine he has SOME inkling of what he wants to do in the future, even if that is highly dependent upon how the rest of the season turns out), it's clearly something the team needs to be thinking about right now. Smith is a major part of this defense, and his potential requirement could have consequences for how this defense will play in the future. With important players on the defense missing time this season, Smith provided a huge degree of stability in the front seven, especially early on in the season.
As Barrows notes, Smith has 36 more tackles than any other defensive lineman. With five sacks, he is tied with Ahmad Brooks and Aaron Lynch for the team lead. It's clear from both a statistical standpoint and based upon the eyeball test that Smith continues to play at a high level.
But, this is the best he has played in a couple of years. His age may have caught up to him that last couple of season, during the course of which he struggled with injuries that never took him off the field, but did seem to affect his play. Smith has started taking Wednesday and Thursday practices off, opting to work out by himself while his teammates have practice. Given Smith's football acumen and his experience, I don't think practice is necessary; but, this time off might foreshadow some imminent problems.
For what it's worth, I think Smith has it in him to continue playing at an elite level. The guy is a monster on the field, and if he has the drive still that he has played with for the last 100 years or so, then he should be able to keep himself in good enough shape for a couple more seasons. This, of course, assumes that he wants to come back; and if he wants to retire, then good for him. He's earned it. But, the team needs to be thinking about this, and they need to be thinking about it now, even if Smith isn't.