D whistles - either high or low whistles - generally are the starting point because they tend to offer the ability to play in more social/common keys of music, which comes into it's own when playtign music with other people.
The usual route is to learn a little on a high D whistle to learn breath control, the basic fingering patterns and some ornamenation.  This can then be taken forward to low whistles to get comfortable with covering toneholes while making use of the skills developed on the high D.  Some people are however much more motivated to learn low whistles than high whistles, and there's not any particular reason not to start on low whistles.  So like many things there is no definitive answer to this question.
In terms of which keys of low whistle to start on, people do often start on Low D because it is the original Low Whistle. This means developing suppleness in the hands early on, which then makes jumping on the smaller whistles between the high and low D easy.  That said, again there are no rules here, and it is possible to learn in between keys first.
Generally though, in most genres, if you will be playing music with friends and musicians, it's good to give preference to the High/Low D initally, and then augment the range of keys you can play in at a later date.