First, there is no free travel for seniors on MTR in Hong Kong. Persons over 65 are qualified for concession fare, which is about half-price. You will be buying an "Elder Octopus card". But no concession on the Airport Express - you'll be paying full fare, either with regular ticket, or if you use your Elder Octopus. And you may still be asked to show proof of age when you board whatever transportation, especially if you look youthful.

On Wednesday, Saturday and official Public Holidays, it's HK$2 per trip on the MTR (not including Airport Express) for seniors. On Sundays and official Public Holidays, most bus routes cost HK$2 or half-price (whichever is less), but not all. Most public transportation offer free trips for seniors on the upcoming Elder Day on Nov 20, 2011. By 2nd half of next year, the government may start subsidizing all seniors to ride most public transportation for HK$2 at all times.

Back to your question about flying through Shenzhen (SZX). Generally, the easiest way (other than taking a private van) to get there from Hong Kong is by the taking the ChinaLink bus. It departs from the Elements mall above the Kowloon MTR/Airport Express station, and first bus departs 6am, with service every 20-30 minutes, though I have seen them adding extra buses when full. Here's their timetable in Chinese:

http://www.chinalink.hk/co_service_routes16.html

Not hard to understand. Third time column is departure time from Elements (across from the Starbucks in the mall, very easy to find). First column is if you board their connection bus from Wan Chai, or second one from Jordan. Though I highly recommend you make your way to Elements and board directly from there. Journey takes about 90 minutes (even if they say 75), HK$110 each way, no concession. Since first bus departs at 6am, you can get to SZX as early as 7:30a, which is okay for flights as early as about 8:45a, though you won't have much margin of error. Book something a bit later if possible.

Note that the 90-minute travel time is approximate. You'll be getting off the bus once and taking all your belongings to go through both HK exit immigration and Chinese entry immigration at Shenzhen Bay, then reboard for the reminder of the trip to SZX. If it's preceding major holidays in Hong Kong or China, you may spend 90 minutes just to go through Chinese immigration, which I have experience personally.

Thank you for the detailed reply.

Cheers, Alan