Sonoma County's workforce is aging at an unprecedented rate and teachers are at the fore with the largest concentration of older workers of any occupation.
More than one-third of the county's educators are 55 or older and age-eligible to retire, well ahead of real estate, health care and social assistance, according to a 2013 economic indicators report from the Sonoma County Development Board.
The wave of potential retirements comes at the same time that colleges and universities are turning out significantly fewer teacher candidates from their initial credentialing programs.
"We are just going through into uncharted waters," said Ben Stone, executive director of the board. "I don't think any culture in history has gone through this kind of demographic shift."
Across California, more than 30 percent of teachers are 50 to 62 years old. Slightly more than 4 percent are 63 years or older.
"I do know that I have been told that if you have more than 25 percent of staff transitioning at any one particular time, that gets to be problematic," said Wally Holbrook, superintendent of the Lake County Office of Education where 35 percent of the teaching ranks is age-eligible to retire.
Experts caution that retirements are hard to anticipate because while 55 is the retirement threshold, many teachers don't qualify for maximum retirement benefits until about seven years later.
And just because a teacher is "age eligible" to retire doesn't mean he will or should, said Jeff Heller, assistant superintendent of human resources at the Sonoma County Office of Education.
"That institutional knowledge is something that you can't replace," he said. "That is definitely a concern when you have those huge numbers."
The percentage of teachers who were age-eligible to retire in Sonoma County hit a peak last year at 37 percent. This year, the percentage dropped to 31 percent, according to Heller.