Is this crack in tire something to worry about or ok defer replacement?
Looks like a lot of nothing to worry about.
Not for the crack but I will check the age of the tire-next to DOT with week/year of manufacturing make sure it is not antique.
For a single crack, that would be OK, but if there are multiple cracks like that, the tire is probably too old for further use.
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I agree. 
It might be worthwhile to have the tire removed from the wheel and inspected internally. The crack may be the result of impacting a pot hole, etc., and there might be damage to the belts or cord body.
As a matter of fact, as I took a second look at the photo I see what might be a crack in the wheel that aligns with the crack in the tire.
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Nice catch, Rod. I enlarged the photo and that certainly looks like a crack to me.
Follow Rodās advice. 
I agree with other posters. I just want to add that the manufacturing date is a 4-digit number. For a tire that was produced last week, the number would be 4716 for 47th week of 2016. If the tire is more than 6-8 years old, I would begin to worry a little.
Looks like not a big problem unless thereās air coming or you can see any cord. But if you are really worried, have a tire shop inspect it for you.
Me, I wouldnāt drive at high speed on the freeway w/ that tire. Around town driving probably ok.
Agreed that it looks like the wheel hit something. Curb or whatever. Note the scuff on the tire.
I tend to agree with GeorgeSanJose. Around town would be ok but Iād be a bit antsy over it on highway driving and with the split right at the bend into the sidewall.
That is a very small crack. Modern tires deteriorate in a short period compered to the tires of 25 years ago. In a few years the cracks on those tires will likely be excessive.
Wow, Nevada, that tire has some serious issues going on. Looks like one of them is old age. I donāt think Iād want to drive on pavement on that one. 
Iād also guess that that tire has seen a lot of off road useā¦ rock climbing?
As a matter of fact, as I took a second look at the photo I see what might be a crack in the wheel that aligns with the crack in the tire.
that got me worried, turns out that isnāt a crack, the photo isnāt clear since itās not likely original resolution after upload, and I may have zoomed in for the photo originally.
What looks like a crack at the wheel (cover) below the tire crack is actually just a āfinā of the tire, not sure what you call them, the tiny points that stick out of the tire like the tiny spikes of a rubber sea urchin kind of toy/massager. There are several of those along the side of my tire, it just so happens one of them is aligned to where the tire crack is.
As for tire age, theyāre labeled 1812 for 3 of them, and one 1913. I bought them in 2013. So theyāre not that old.
FWIW the tiny āpinsā on tires are sipes. They are the result of vents called sipes in the mold to allow air to be expelled. Once the air escapes uncured rubber is pushed in where it cures, most is trimmed off.
Nevada_545 said: That is a very small crack. Modern tires deteriorate in a short period compered to the tires of 25 years ago. In a few years the cracks on those tires will likely be excessive. [[Photo omitted]]
Mountain Bike is right - There is more going on in the photo he provided than just a crack. The tire is in the midst of a tread separation and the cracks are merely incidentals to the process.
Sorry, my friend, but thatās incorrect.
I can give you a dozen more if youād like.
In the old days āsipingā was an aftermarket process done by all tire stores for extra cost. Theyād slit the tread blocks on our bias ply tires to provide more edges to bite the ice and to make the blocks a bit more compliant. It worked.
The little nipples are called āsprue marksā. They can be caused by the material release paths or the material injection paths.
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Instead of sprue, The tire industry uses the term āventā - or at least the part I work in does.
Cool. I learn something new every day that Iām willing.
Actually, from a purely technical standpoint, only the feed channels are āspruesā but that marks from both are typically called āsprue marksā.
As Mr.Firestone said to Mr. Goodyear, āWhatās āsprueā with you?ā