May a juvenile adjudication for theft be used as an "enhancement" conviction for the purposes of 31.03(e)(4)(D)? I have a 20 year old thief with one good adult theft conviction and one juvenile theft adjudication. Sure would like to make this one a felony. Couldn't find any caselaw in my quick search.

Absent specific authorization for it the answer is no. The family code provides in two different sections that juvenile adjudications aren't convictions. Thus, unless the penal code says that a juvenile adjudication works, e.g. Sec. 12.42(f), it does not.