From LASORS 2007 on the CAA website:
An applicant for a JAR-FCL PPL(A) is required to pass
theoretical knowledge examinations in the following
subjects:
1. Aviation Law & Operational Procedures
2. Human Performance & Limitations
3. Navigation & Radio Aids
4. Meteorology
5. Aircraft (General) & Principles of Flight
6. Flight Performance & Planning
7. JAR-FCL Communications (PPL)
These examinations contain multiple-choice
questions for the most part and are normally
conducted under the auspices of a Flight
Training Organisation or a Registered Facility.
An applicant shall be deemed to have
successfully completed the theoretical
examinations for the JAR-FCL PPL(A) when
awarded a pass in all of the above
examinations within a period of 18 months
counted from the end of the calendar month
when the applicant first attempted the
examination. A pass will be accepted for the
grant of a JAR-FCL PPL(A) during the 24
months from the date of successfully
completing all of the theoretical knowledge
examinations.
You must have passed ALL the theory papers within 18 months of taking the FIRST. The skills test must then be taken within 2 years of completing the LAST paper.
There is no set order, although most people take Air Law first (and some clubs / schools may have their own requirements for solo flying as a student).
And yes, you must be 17 to take the skills test!
Lasors:
An applicant for a JAR-FCL PPL(A) shall be at least 17
years of age but some of the required qualifications for
the grant of the licence may be gained earlier.
Applicants should ensure that any qualifications gained
earlier will still be valid at the time they plan to apply for
the grant of the licence.
Student pilots may act as Pilot-in-Command from their
16th birthday provided they act only in accordance with
instructions given by a flying instructor, hold a valid
JAR-FCL Medical Certificate and, generally, fly only in
UK territorial airspace. There is no minimum age for
dual instruction, but any received before the age of 14
is not countable towards the experience requirements
specified in this document.
Good luck!
SD