What factors determine the height of the turbopause?
A major consideration for any such altitude measurement, hence determination of the factors is, according to Turbopause determination, climatology, and climatic trends using
medium frequency radars at 52°N and 70°N (Hall et al. 2008), that


  measurement of the turbopause altitude is a rather different matter, primarily because direct measurements of small-scale fluctuations in neutral density, temperature or motion at altitudes of around 100km are virtually impossible. 


The authors state that seasonal variation plays a significant role; the article The “wave turbopause” (Offerman et al. 2007) elaborate on this in their abstract:


  Substantial seasonal and latitudinal variations are found, with some interannual variability also present. Seasonal changes are annual at high latitudes, semi-annual at low latitudes, and a mixture of both at middle latitudes. Southern hemisphere data are similar as in the North if shifted by half a year. Latitudinal variations show a minimum in the tropics and two relative maxima at middle latitudes.


With observations made in northern Norway in the article Seasonal variation of the turbopause' One year of turbulence 
investigation at 69°N by the joint University of Tromso/University of Saskatchewan MF radar (Hall et al. 1998) showing that the levels at these latitudes decreasing in summer months.

However, long term changes seem to be occurring at high latitudes, with the Hall et al. (2008) observing


  the classical radar turbopause has been at constant altitude at 52°N during the last decade. At 70°N, on the other hand it appears to be moving upward at a rate of between 2 and 7 km decade$^{−1}$.


Although the authors suggest that there may be an anthropogenic cause, the observed reasons for the high latitude change appears to be due to:


  the trend we report at 70°N is commensurable with an observed cooling of the mesosphere (and in turn a lowering of the ionospheric E region altitude) and an increasing average electron density in the ionospheric D-region.