Ukraine

Is it safe to assume the vast majority of Russian soldiers are males between 19-30?

If so, based on the population breakdown that I looked up, Russias casualties are just under 10% of their entire population in that demographic in the first year of this conflict.
No, they skew older, similar to Ukraine. While they do have mandatory conscription for younger men (18-25), with deferments for education / etc., my understanding is this "boot camp" is largely a joke - even heard this before the war. That they mostly just do some PT, march around, get hazed, and do unskilled / hard labor like fixing up the commanders house. The weapons training is very scarce. You might go the range and actually fire a mag or so, but thats it.

These conscripts technically are a reserve / defense force that cannot deploy outside of Russia. In order to do so, they have to sign a contract. Now while Putin did annex large areas of Ukraine, so they are technically Russia and they could legally be sent there I believe there is political pressure for him not to do so.

But OTOH I'm sure there is a great deal of pressure on these conscripts to sign contracts which allow to be deployed. So it comes down to who yo daddy is, kind of like Vietnam.