Undervoltage lockout
This is the simplest circuit I can think of using BJTs that will provide under-voltage lockout.



simulate this circuit &ndash; Schematic created using CircuitLab
How it works:
If Vin is higher than the the Zener voltage of D1 plus 1 diode drop (Vbe of Q1) then D1 will conduct, providing base current for Q1. Q1 will conduct providing base current for Q2, and Vin - Vce(Q2) will appear at Vout.
If however, Vin is lower than the Zener voltage of D1 plus 1 diode drop, then D1 will not conduct appreciably, Q1 will not provide base current for Q2, and Vout will float.
R1 and R2 serve to limit the current through the bases of the BJTs.
The Zener and BJTs should be specified to have low leakage current. If the Zener does not, you may need a resistor from the base of Q1 to ground (to divert leakage current).
Note, this circuit has no hysteresis, so if Vin remains close to the threshold voltage, the output may/will bounce on and off.