I would avoid telling the agency very much at all regarding preferences based on age (think about age discrimination). Instead, tell the agency how much you are prepared to pay (always tell the agency an amount which is less than your actual estimated amount, so that if you get a super applicant, you can offer more). Make sure you understand Tax/NI and Employers NI. Offer only a GROSS salary.
Be very clear with the agency regarding the duties the nanny would have - especially domestic duties - be specific... don't just say 'nursery duties' as I feel that means little. If you want someone to clean, then specify that to the agency. Hours of work are also important, be specific in the working hours if you can, or at least give a reasonable estimate - such as 11 hours per day, or whatever. Also make sure the agency understands that the job is not SOLE CHARGE - as you don't want applicants who only want sole charge, when you will in fact be at home some of the time.
Aim to see a range of applicants, some with baby experience already, others who have more general experience. While specific experience is desirable, the best person will be whomever gels with you, who understands your needs and your child's needs... it often falls down to relationships!
If it is a live-out position, one thing to specify would be how far away someone can live from you... as you really want someone local, not someone who has an hour commute both ways.