Recently found this pre-referendum survey:
http://avaazpress.s3.amazonaws.com/558_Crimea.Referendum.Poll.GfK.pdf.
What is interesting:
1. The younger age group, the lower number of those who self-identified as Ukrainians. This means that in Crimea Russians assimilate Ukrainians, not vice versa (not very surprising taking in mind overwhelming prevalence of Russian language and Russian culture).
2. However, in the oldest age group more people voted for joining with Russia, and if option "Leave status of Crimea the same as it was before February 2014" would be available, it would be more popular in the youngest age group and less popular in the oldest one. Nothing surprising too: elder people tend to be more nostalgic of Soviet past when Russia and Crimea were parts of a single country, and many of them were even born in Russia (then RSFSR) before Crimea was transferred to Ukraine, while those whom are 18-35 years were raised or even born in independent Ukraine and relatively larger number of them are ideologically liberal (i. e. pro-Western, pro-EU and critical of Russian political regime). And share of Crimean Tatars is very low in the oldest age group, it certainly contributed too.
3. Most of those who was polled were satisfied with only 2 voting options and believed that presence of Russian troops don't influence how freely people vote.
4. Results in Sevastopol, where Russian influence always was especially strong and Crimean Tatar population is negligible, aren't as different from other Crimea as I would expect. Difference between Simferopol and Sevastopol is less than a margin of error.
Unfortunately survey don't include data by age group, it would be especially interesting.