If units have already damage on them from a spell, and also no showdown takes place to heal them. Then opponent plays elder, do the damaged units die immediatly from elder static ("any damage is enough to kill units") ? Or the pre-existing damage doesnt matter

Yes — the pre-existing damage matters, and those units die the moment Elder Dragon hits the board (provided that damage was dealt by Elder Dragon's controller).

Elder Dragon's static doesn't deal damage; it redefines lethal damage for units carrying damage marked by its controller, so any non-zero amount is lethal (Rules 142.4.c, 323.5). Spell damage stays marked until the end of turn — it only heals after combat or during the end-of-turn cleanup — so it's still sitting there when the Dragon enters, and a cleanup runs before anyone gets priority to react to the "when you play me" trigger (FAQ 10686, FAQ 11876). No showdown or combat in between makes no difference; if anything, it just means nothing healed the damage away.

The one real condition: "your damage" is relative to the Dragon's controller. If the earlier spell was cast by the Dragon player's opponent, that damage is not theirs, and the passive won't make it lethal.