What RF is saying is in general terms, theres no hard and fast rule that says you have to have had 7 years of experience, etc, the key point is that you must be competant. Oh and inspection work is quite a different skill from installation work. There is I beleive a member of the IET forum who is soley in the I&T sector and whos background is of a engineer for the post office, with the mail sorting machines, etc, rather than a sparks as such.
Its all about having the right approach to inspection, and as rob says certainly having guidence from someone who has carried out a lot of big PIRs is needed, the process is quite different from testing as part of initial verfication, or even PIRing a domestic
I personally got into I&T work pretty early on in my carreer (less than 7 years), but I did have a good knowledge of BS7671, a good mentor and a sound understanding of safe live testing practices [as well as knowledge of what to do if things don't quite go to plan*]
Rob may want to disagree, but I will maintain that I consider myself better placed to carry out a large PIR than someone who has done 10 years of house bashing/ newbuilds and nothing else etc.
Remember PIRs are something that the industry does poorly generally, normally caused by a desire to full the page of test results rather than inspecting stuff... you can have pages of fully tested circuits, but if you haven't walked around the area and noted that the bonding is missing, the trunking has been torn off the wall by the forklift, the db is missing 5 blanks, the door switchfuse has asbestos flashpads, the hi-bay is strapped up with 6491x and the busbar chamber has a 50mm hole in the top, then its beside the point really
(Ok, i've not quite seen all of them... very nearly though**!)
* Old installations can habour supprises, whether its the light switch that the conductors fall out of when you pull it forward, or borrowed neutrals not nice when it happens, but need to be able to deal with it in an appropiate manner
** It was the trunking that was strapped up with 6491x, the Hi-bay had been smacked with a JCB - it wasn't that high afterall