I would think you could expect anywhere from 12K to 16K at a language inst., working full time. The economy is picking up here, so those figures may change (hopefully higher!!), but really if you can sell yourself, and the school really likes you, who knows?! Keep in mind that a NET teacher (government sponsored in a public school) will start at around 16K plus living allowance. I would suggest you focus on a kindergarten, if you're here for the short term. You can start full time at 16-17k, and often you don't need a degree (I didn't). You'd prob. be teaching phonics (not what you learned in TESOL, I know) but easy work, and if you love kids, its alot of fun. On top of a full time job, you can tutor privately in the evenings as well, and really make some cash. There you could prob. charge 200/hr, in the right areas of town. I'm not sure where you live, but HK is a pretty expensive city to live in, so bear that in mind.
I can't say l've notice age limits....have you tried www.classifiedpost.com for jobs? Lots of language institutes there, as well as the careers page in asiaxpat.com.