what age should i worm a chicken.

"What age should I worm a chicken" you ask.  Judging from your photo I should wait till you are a bit older or get your mum or Dad to do it when the birds are about 12 weeks old.   There after  a good regime is worm four months apart but do an extra fourth worm in between in mid summer (eg July, Sept, Jan, May).  If you dust with louse powder I'd recommend you use diatomaceous earth (DIATOM is a brand name) in their nest boxes/ dust baths and on the hen rather than the repellant powders such as Barrier. These are only intended to be dusted in chickens not houses and not in my opinion effective.  You may as well just shred the £10 note and use that as it's likely to be as effective.  Incidentally the active ingredient in Barrier's Red Mite or Louse powder as well as their fly sprays for horses is based on a fly/ Mosquito repellent "Citriodiol"the manufacturers of which haven't tested it against and don't claim it to be effective against mites. Though there's good evidence it works for flies and mosquitoes on humans.  I don't completely trust testimonial evidence of effectiveness of treatments which aren't also backed up by scientific clinical evidence of efficacy and unfortunately the amateur animal medication market is rampant with such products.
If you need a treatment for a louse infestation use Invectin or Frontline drops on the back of their neck otherwise a good preventative is to ensure they have a dust bath to use which can be sprinkled with Diatom or other similar product.  Ash is a possible suitable substitute. Diatomaceous earth is non toxic and works (it is claimed) by its purely mechanical properties of abrasion and dehydration.  There's good anocdotal evidence it works though as far as I'm concerned the juries out though in my own trials the results seem promising so far.  It's relatively cost effective Barrier powder cost £9 - £11 for 100grams whereas Diatomaceous earth will set you back 10p / 100grams if you buy it in 25kg sacks though it is available in 1KG tubs at much higher cost but still no where near Barriers costs. 

I'd be interested in others views on this topic as I think we're being ripped off something chronic by claims of miracle cures for this or that and it's really a lot of quackery and not only the duck treatments.

Regards