What are the best options for securing long-term residency in Thailand for a couple with an age difference and approaching retirement age?

Another option if you don't mind paying is the "Thailand Elite Visa" You can get more info on the cost here:

As others have said there are 4 ways you could go

First two are whilst in Australia you apply for either an OA or OX visa. They can only be done whilst inside Australia.

OA is for one year plus one so 2 years just before your first year ends your fly out of Thailand then fly back in and get stamped for an extra year

OX is for 5 plus 5 years. On entry into Thailand you are stamped in for 5 years at end of 5 years once again you fly out then back and get another 5 years

Both of these you can do through Thai Consulate in Sydney

Or you can enter Thailand on either a 30 day visa excempt and get an extension on it which will give you 60 days in this time you visit an Immigration Office and get a print out of what is needed for this visa, your first things is to open a Thai bank account and deposit the money needed

Or look at the Elite Visa

My husband and I are Australians and we have been on the OA a couple of years ago and now on the OX if you would like to message me I can explain it more and can find out how friends on an OA visa did his wife as a trailing wife

If using the trailing spouse route then look up how to get your marriage certificate legalised for use in Thailand. Different methods for different nationalities & might be easier in your home country

Also check out the "Thai Elite" visa.

wife can become a dependent on your retirement visa until turning 50 i had same issue when we first came here had just turned 50 but wife 11 yrs younger

When you are 50 you can get a “retirement visa”. That’s in quotes because it doesn’t exist. There’s numerous visas that have that monicker but you’re basically looking at either a Non O visa based on being over 50 or a Non OA or Non OX with the latter two only available from your home country.

If you were to obtain one of these your wife would be able to piggyback off of that as a “trailing spouse”.

A caveat to that is that they won’t allow her to obtain that visa in country. She would have to visit a local embassy/consulate in a nearby country to get it.

Before you turn 50 there’s few options. Ed visas are possible but they’re clamping down on those as they were wildly abused. Volunteer visas even rarer these days. If money isn’t an object the the Elite visa could work for both of you. Off hand I don’t recall the price of the family (2 people) one but guessing around 800k for a 5 year visa for both. Forgive me if I’m wildly off the mark on that price as I haven’t looked it up in ages.