Google's data implies a total smartphone user base of 317 mn. Per Google's Consumer Barometer which tracks certain demographic trends with respect to internet usage, ~55% of population below 25 years of age used smartphones to access the internet in early 2015, compared to only 11% of population above 55 years of age.
As per Kotak's estimates [Derived from talking to Telcos - Airtel, Vodafone Idea] , users below 35 years of age comprised ~78% of the total smartphone user base as of Mar?15; we believe this proportion will go down to ~64% by FY2020E. The under-35 segment will still remain the dominant segment, but Indian smartphone user base will become more broad-based.