When it comes to software update and roll out speed, the iOS has always had an upper hand over android. While a new iOS version might roll out to majority of compatible iPhones in a matter of days or weeks, a new android version might take months or even years before it gets to majority of compatible devices.
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The simple reason for this is that while iOS has only iPhones to feed, android OS has to be ported to different OEMs.

Even with the relatively slower roll out of the iOS 11 compared to previous versions, the latest iOS is currently running on over 60% of all iPhones globally while the latest android Oreo update is running on less than 1% of all android phones.

This has always been a thing about android, if you need swift OS update then you need to get a Google phone because even the top android brands like Samsung can't even offer you the latest OS version until after a couple of months of release.
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According to the most recent distribution numbers published by Google for the month of December, the 2 years old android marshmallow is still the most used android version with 29.7%, closely followed by 3 years old lollipop with 26.3%. You can check out the full distribution below.

Android Gingerbread - 0.4%
Android Ice Cream Sandwich – 0.5%
Android Jelly Bean – 5.9%
Android Kit Kat – 13.4%
Android Lollipop – 26.3%
Android Marshmallow – 29.7%
Android Nougat – 23.3%
Android Oreo – 0.5%

Note that the distribution you are seeing above is compiled from all the Android devices that have visited Google Play Store during a seven-day period ending December 11th.