To apply Noether's theorem, which is what you are alluding to here, one needs to look at continuous symmetries of a Lagrangian description of a system's dynamics.
The damped oscillation equation you have written, although it is invariant with respect to a time translation as you rightly say, is not a Lagrangian description. If you write the Lagrangian for this system, you'll find that it is not time shift invariant, so this lack of symmetry is where your implied argument ("my description does not depend on time $\rightarrow$ energy is conserved") breaks down.
Some dissipative systems have Lagrangian descriptions but they always wind up with time varying Lagrangians, so there is no contradiction of Noether's theorem: See Joseph Johnson's answer and link to read about some interesting historical attempts to broaden the Lagrangian notion to all systems here.
Incidentally, if you want to model this kind of thing in quantum mechanics, a way to do this is to embed your oscillator in a huge set of coupled quantum mechanical oscillators where your initially excited oscillator is coupled weakly to all the others. The system as a whole varies unitarily with time, so there is no overall dissipation, but if one oscillator begins in a raised state with all others in their ground state, the amplitude to find the oscillator in its excited state dwindles exponentially with time and the energy becomes inexorably spread throughout the whole system of oscillators - a lovely illustration of a simple macroscopically "irreversible" (but microscopically reversible) system. You can do the same in theory with a classical harmonic oscillator too - embed it in a huge system of oscillators and couple it weakly to a huge reservoir of others: the same thing happens. The oscillator on its own follows a damped equation, even though the whole system is not dissipative.