The upper age limit for TA officers has been extended to 60. This is not, however, a guarantee of service to that age. For an officer to continue serving beyond the previous upper age limit for his arm/service he will have to apply for and be accepted into a relevant post which does not block the career progression of a younger officer. If there is no post available the officer will have to retire. At this point I do not believe they will be able to re engage at a later date.
I have heard of increased age limits for non commissioned ranks in certain key trdaes (especially medical) but have not seen any details. I would hazard a guess that any such extensions would be on a similar basis to that above.
I have just finished a course where a chap is just re-entering at the age of (I think) 60 (I kid you not!) He has loads of previous time in uniform, and impressed all us 'younger' new recruits by having para wings and by previously having been TA SAS However, he impressed us all a lot more when he trashed us all in the physical!!!
I think the truth is that nobody knows. Last year when I was investigating joining the TA I was told variously that the upper age limit was 38, 40 and 44 by three different sergeants at recruiting offices. I was then told 40 to an email enquiry to the Army, and it was only by chance that I was talking to a senior chaplain who told me to go for it even though I was 43. I think it depends a bit on your previous and your trade. It would be a good idea though if the recruiters could all sing of the same hymn sheet.