At the Brisbane Heavy Maintenance facility (Qantas 767/a330 maintenance), the oldest apprentice this year is 61 years old. Actually only around half are under 21. Common ages here in the Brisbane facility for apprentices would be 20's and 30's. For the intake of around 20 to 25 people (all inclusive of structures, avionics and mechanical) this year, around 6 to 8 would be aged 30 plus.
But Saintsman, I understand what your saying. If you had to get a plumbing, sparky, chippy apprenticeship (ie. building trade) with a small business, it would be hopeless unless your 17 to 20 years old.
To get into Qantas these days, its pretty much necessary to do a pre-apprentice course, with the Aviation Australia course been the most popular. This is somewhat risky as they turn out way more graduates then apprentice positions available.
Hope this helps,
cheers.