As one who started on viola I offer the following perspective : I rather believe that the first 2-3 years of study can be effective for learning basic techniques (up to 5th position) on either instrument. The principal difference is in perspective as a child or adolescent. I believe that a young violist is rarely (if ever) star-struck with his or her own brilliance as are far too many violin-learners. Too many of the latter want to dash into Paganini without nearly enough experience. Also, violists are more naturally attracted to chamber music and ensemble playing, and learn early the importance of the middle parts which violinists may take years (or even an entire career) to appreciate.
I must admit that I dearly love both instruments and play about 50-50 on each, so am not really prejudiced. I just think that the violin produces far too many egotistical young prima-donnas while violists become more seasoned musicians earlier just by the nature of the instrument's role.
In sum, a young person should play whatever they truly are interested in (even if that should include, [gasp], drums, banjos, or accordions)!
C-V