Shapechange question

the language of the shapechange spell reads "..You must have seen the sort of creature before, and it can't be a Construct or an Undead."

my questions is: if a player has seen a dragon of a certain age, say young, can they then shapechange into that dragon at a different age?

bonus question:  if a player uses shapechange to change into a creature, and another player sees them, does that count as having seen one to then have that second player also shapechange into it as well?