Ask Mr. Smarty Plants

There are lots of plants with nightshade as part of their common names; but if you mean Solanum americanum (American black nightshade), I wouldn't try it if I were you.   You might not have the chance to say "I am 19!"   [Please note that Solanum nigrum is a synonym for Solanum americanum.]  See the links from Poisonous Plants of North Carolina, Canadian Poisonous Plants Information System and the University of Pennsylvania Poisonous Plants Database  for information about Solanum americanum.  Most of the other nightshades are also in the Genus Solanum and most are noted for some degree of toxicity.  Here are other nightshades that occur in Oklahoma:

S. citrullifolium (Watermelon nightshade).  Native.  Here are photos.  I could find no record that it is toxic.

S. dulcamara (Climbing nightshade).  Non-native.  Here are photos and more information from Noxious Weeds, King County, Washington.  Charecterized as HIGHLY TOXIC by Poisonous Plants of North Carolina and it is listed in the Canadian Poisonous Plants Information System and University of Pennsylvania Poisonous Plants Database.

S. elaeagnifolium (Silverleaf nightshade).  Native.  According to Southeastern Arizona Wildflower these plant, especially leaves and green fruit, are poisonous.  However, the Veterinary Medicine Library of the University of Illinois says that for S. eleagnifolium it is the ripe berries that are more poisonous than the green unripe berries—this is in contrast to most other species of nightshade.  They also mention that silver nightshade can be poisonous at only 0.1% of the body weight.  The Veterinary Medicine Library of the University of Illinois lists all Solanum species and says that risk of poisoning depends on the species, the maturity of the plants and other conditions.

S. interius (Deadly nightshade).  Native.  Here are photos.  I could find no source listing S. interius as toxic.

S. physalifolium (Hoe nightshade).  Non-native.  Here are photos and more information from Montana Plant Life that lists it as potentially toxic.

S. ptycanthum (Eastern black nightshade).  Native.  Here are photos and more information from Michigan State University Extension Service.  Listed as toxic in the Plants of Texas Rangelands.

S. rostratum (Buffalobur nightshade).  Native.  Here are photos and more information from Southeastern Arizona Wildflowers.  Listed as toxic to cattle in the Plants of Texas Rangelands.

S. triflorum (Cutleaf nightshade).  Native.  Here are photos and more information from Montana Plant Life which lists it as mildly toxic.

University of Illinois Plants Toxic to Animals lists several of the species of Solanum.

Another plant with the common name of nightshade, not in the Genus Solanum, that occurs in Oklahoma is Circaea lutetiana (Broadleaf enchanter's nightshade).  This plant doesn't appear in any of the poisonous plant databases that I accessed.  It isn't "especially toxic" according to this Wikipedia article.

There are notable exceptions to the toxicity of the Solanum species—the very edible Solanum tuberosum (Irish potato), Solanum melongena (Eggplant) and Solanum lycopersicum (garden tomato) are in the Genus Solanum.   However, even the Irish potato can have some toxicity.  Potatoes that grow too near the surface of the soil and are exposed to sunshine sometimes turn green.   The green portion is potentially toxic.

So, if I were you, I would forego eating the berries or leaves of any of the species of Solanum with the common name of "nightshade".  You might not be able to distinguish between the more deadly ones and the only mildly toxic ones and that would be a risk I certainly wouldn't want to take myself!