Wow, this is interesting.
What fish are going to stay in the Brackish tank?
In answer to the questions
1) There is no documented age as such, it depends on how close to brackish water they are born in the first place.
2) To bring a tank to brackish, above SG 1.009 is a MUST for these fish, it must be done very very slowly over several weeks, even a drip system is too fast for them. Raising the SG too fast will kill the fish. You need to know whether the fish are kept in brackish at the store. THIS IS IMPORTANT, if they are, they MUST go into a brackish tank immediately, otherwise putting them into freshwater will kill them.
3) The bacteria will once you start adding the salt water, gradually change to the saltwater variety, this is another reason to very slowly change the salinity, do it too fast and it will cycle the tank, slow allows the bacteria to adapt to the conditions the salt is creating.
4) The test kits are identical for fresh and saltwater, what is different is the color cards showing the levels. You can download the saltwater charts from here
If you are cycling the tank as a freshwater then you need to use the freshwater test kit, once you start adding salt, you will need to use the saltwater kit once the SG gets over 1.002. You will also need ideally a refractometer ($40 on ebay or from your local store) to test the salinity level.