Hey there, I may be able to help you with some of your questions:

1. I'm not sure how long have you been working in other firm, but let say if you have been working for a year, then you should definitely try applying. Please bear in mind that there's no 100% chance for your to be landed with an offer, it all boils down to your performance during the interview as well as your English writing skills (which is one of the crucial skills required in the field of TP).

2. In my opinion, you shouldn't even be applying in the first place if you can only start work in September. We would expect new joiners to start work within a month or two, especially for junior positions. You should only apply when you are ready to leave your current company, say in around June / July and serve your notice period in August (if you have a one month notice period). In terms of age, nope, age is not a concern, haha TP departments are in need of capable new team members.

3. CPTX experience is not too crucial when it comes to TP work, but it revolves more on your business and economic sense and your writing skills (you'll need to be able to explain a particular scenarios in a clear and simple manner with words). You'll probably only be using on your CPTX knowledge when it comes to TP / tax audit cases, but not so much in your normal TP engagement. Trust me the nature of work that you'll be doing in TP will be very different from CPTX work.

4. Stress - All departments within the big4 definitely will come with a certain level of stress. To give you a clear picture, perhaps let me provide you with some examples of the situations that you may come across in a TP department:

a. As our TP projects are driven by clients' internal deadlines, thus we could be busy for the whole year round and we do not have a peak period per say.
b. During busy period, there would be times where you have to manage few on-going projects at the same time with very tight deadlines. Hence, this is where a good project management and time management skills come into the picture.
c. For tax / TP audit cases, it could get very stressful due to super tight deadline. In addition to this, you will to deal with your clients (to guide and get them to provide you with the information required), preparing the relevant cover letters and the necessary tax exposures, which all you have to do at the same time.
d. Occasionally, there could urgent and ad-hoc engagements due to new regulations which may require you to deliver in a short time.

Hence, it is important to have good project, time and stress management skills. Hope the above, will provide you with some pictures that you may be facing in a TP department and wish you all the best in your career.