To learn fixed gear, better start with high gear and then get lower, or the opposite? I am wondering if it is better to start to ride a fixie with a relatively low gear, and as experience comes, switch to a higher gear, or if the advised practice would be to start with a higher gear, and then perhaps change it to a lower gear.

I know you need to be strong to go uphill with a high gear, but you also need experience to go downhill fast with a low gear, so which would be considered more "expert"?

EDIT: I am not planning to ride on a track, rather on open road with rolling terrain and possibility of hills (not extra-steep, I care about my knees).
In general I would recommend starting low. Your legs will go through an adjustment period and you may find that your knees get sore from a larger gear. After you've ridden for a few weeks pay attention to how you're riding. If you're in a hilly area are you struggling to get up the hills? Do you find that your cadence is regularly high enough that your hips are bouncing on the saddle? There are both indications that you should adjust your gearing high or low respectively. 

It's can be cheaper to replace a track cog than a front chain ring so start with 16T in the back and then pick a chainring that gets you into the gear you're looking for. With any luck, if you do need to make an adjustment up or down switching the rear cog won't force you to change your chain length.