I would have to warn against treating survey subsamples as gospel truth, particularly when said samples are rather small.
The sample size for the 16-17 age group in the Ashcroft poll is 14.
Yeah, its useless regarding the smallest sub samples. The huge difference between the 16-18 being the most pro-independence and the 18-25 group being the third most pro-union (=strongest under 55) is striking and likely due to a fluke. Then you have 25-34 being the second strongest Yes group. So a thorough study with an adequate sample of the attitudes of Scots aged 15-35 would be really interesting.
Despite small sub samples the data in the Ashcroft poll does support the general picture from previous polls.
A. Scots get more unionist the older they get from around 25 and upwards
B. 65+ are a lot more unionist than any other age group
C. Young people 18-25 are a little less supportive of independence than those aged 25-55.
Its also clear that a majority of Scots under 55-60 (impossible to draw the exact line) voted Yes.
So the interesting thing is if young Scots will keep being more pro-union than those born 1960-1990, and if new generations will be less nationalist, so you get a Quebec style development where nationalism is "contained" in certain generations. I just find that very unlikely in the Scottish case - I think devo max will increase Scotlands feeling of being a separate nation and give appetite for more.
If new generations on the contrary follow the general pattern of the 25-55 old and get progressively more nationalist for each future generation then its obviously over for the unionist cause when the boomer generation dies off.
The declining North Sea oil and gas production will probably make the generational pattern immaterial. With max devo and the SPN holding the policy making reigns, I project that in a couple of decades Scotland will be fairly more dependent on transfers from the rest of UK. And if the Yes weren't really able to make a coherent economic case for independence now, it'll be much tougher with oil revenue being a fraction of what it is.