Hang in there. The begining stages of Hung-Ga training can be trying at times. If the pain you are experiencing is not related to exercise soreness talk to your Sifu & go see a doctor.
Yeah, I echo what everyone else has said (how original) I'm an older dude, too, so I feel your pain (so to speak)!
I'm guessing that Topeng's right about the thighs & forearms. How are your knees and your back? Doing sei ping ma (horsestance) can trigger discomfort if you're not used to the position and/or if your alignment may be incorrect for you. Try some minor adjustments with your Sifu's/Sihing's/Sije's advice and see if it feels better.
Like Jow Yeroc had mentioned, ask your Sifu for some Dit Da Jow (Chinese topical liniment). It does wonders for general sorespots.
Further to what HG1 had said, challenge yourself during your workout, but only to the point of soreness or discomfort, not pain. Myself, I have "old" knees, ankles & a back, but smarter training keeps me going & helps to alleviate those non-Hung Gar-related discomforts. I feel better training those "old bones" then when I don't work them out.
By chance, do you train under Mandarino Sifu? If so, I've always meant to drop-by whenever I've been in the area but something else always came-up
Only advice I can give is train your stance work away from school time. Make sure you stretch everyday, also taking vitamins/fishoils that will help you heal up along with tiger balm, or a good healing solution.