We throw a lot of words around when we talk about training musicians. “Young singers” and “young artists” are used interchangeably for the eager talents that fill apprentice programs around the country. This weekend, the indomitable Wolf Trap Opera offered a few gentle, inadvertent reminders that there’s a difference between the two terms. At “Idomeneo” on Friday night at the Wolf Trap Barns, I heard several talented young singers and one wonderful young artist, Ian Koziara, in the title role.