That really depends on the product. If the aim of your product is to work in third world countries with poor access to technology and internet then you should build something as basic and all supporting as possible. If you're building something without a social edge and it's about monetising from 'richer' consumers then wherever they are they probably have relatively recent browsers.
My last startup was focused on high value customers and we only built it to support latest chrome, last two versions of i.e. (which at the time was 9/10) and safari 8/9.
This site provides a good set of stats:
https://www.netmarketshare.com/browser-market-share.aspx?qprid=2&qpcustomd=0
If you're going for the high end customer make sure you have good support for safari because mac users may be a minority but they spend the most money :)
PS. You should also consider a mobile / responsive version.
