Is current with a frequency less than 1 Hz still considered DC?

AC and DC are relative terms. If you're looking at a 10kHz waveform for 100ns, you will think it is DC. It works the other way around too: if you forget about what's providing you with "DC", who knows if this waveform is not going to change in the next seconds, minutes, days, years? Think the voltage of a capacitor for example during slow discharge. If you monitor the voltage on an oscilloscope, you'll see a flatline. DC you say? Wait longer, and the flatline will decrease in voltage towards zero, which means there is some AC in there as well.

Besides, no signal is actually pure DC, you always have AC components as well due to noise and all sorts of causes. It is only "DC-enough" or "AC-enough" for the application you're intending to use it with/for.

Fourier transforms are a good way to picture what DC and AC components are in a waveform. The transform is constant for periodic signals and depends on time for any non-periodic signals like the capacitor example. For the square wave: (source: wikipedia)

Yes, you can have AC with a frequency less than 1Hz, in the same way you can have numbers between 0 and 1.

Frequency isn't an integer number, but a "real" number. You can quite happily have a waveform of \$1 \times 10^{-100}Hz\$ if you wanted. You'd have to be quite patient to see it change, but it will change, and given time it would trace an AC waveform.

As with any AC voltage, frequency is the inverse of the period in seconds, and vice versa:

$$f = 1 / T$$ $$T = 1 / f$$

As f gets asymptotically close to 0, T correspondingly becomes very large.

As a practical example, I have a function generator that generates any frequency up to 5MHz in 0.01 Hz steps. So at its lowest setting (0.01 Hz), it can generate a sine wave with a period of 100 seconds.