Is it safe for seniors to go for nature walks on dirt trails?

If the senior has approval from his or her doctor to begin or continue a walking program, then by all means, nature walks on a dirt trail are safe.  It may even be easier on the joints than walking on concrete or asphalt.  However, loose dirt and gravel can be more slippery, so you will want to be more cautious on your nature walk.  You will also want to make sure you wear the proper shoes.  There are trail shoes that protect your feet a little better than regular tennis shoes.  The tread provides better traction and is a little firmer than most tennis shoes, but not as restrictive as hiking boots.

Yes and no. Much depends on the level of the seniors in which you are working with. Do these seniors already have ailments that may restrict them from potential rough and uneven surfaces? If this holds to be true then nature walks on dirt trails would not be wise and finding paved nature walks would be best. However, if your group of seniors is very active and if they are injury free then go for it, have fun!