My wife and I are looking to get a Norfolk Terrier puppy and have been researching into playpens as a way to keep him confined to certain areas of the house.

Are playpens generally seen as something to be used only when it's a puppy - i.e. not for adult dogs? Or is it something we could continue to use even when he's older?

If it's generally limited to puppies and shouldn't be used with grown dogs, how can we make sure he doesn't chew furniture, go where he isn't supposed to go, etc when he's older? Is it down to training him from early on?

I use them for the first 12 months and after that pup is mature enough to have freedom of the kitchen/dining area when we are out and at night.   We also use a baby monitor which I really recommend if pup is not going to be in the same room.  If pup is unsettled then I can speak to him on the monitor to calm him and also know if he needs to go out.  Make sure the pen is large enough for his bed and a small play area.  Introduced from day 1 they will take great comfort from it.  In terms of not going where he shouldn't either close doors or put in temporary child gates to prevent access to rooms, stairs etc. Actually plastic trellis works really well at the bottom of stairs also and is very cheap.

Thanks for your advice - that's really helpful! Out of curiosity, what sort of breed do you have?

I use crates for mine especially as pups .I wasn't sure if u meant a crate or a four side pen. Personally I prefer a crate so the dog can come and go and use the crate as it's den/ bed. Used carefully with positive training most actually like.them and can go on to use right through their life .with the door open once u consider pup is safe around the house.
Look up crate training as it needs to be done carefully so it becomes a positive experience and the pup ismt stressed
. At night I would suggest a one by your bed so pup knows u are there and u know if he needs to  go.out when this mean two crates for a while of.course. 
With the exception of one dog mine have always settled happily in their space and were also.fed in them
I've had toy breed , medium gundog breed a fcr and gsd. .it was one of the fcr who just didn't like being in a crate .mainly because she is such a velcro dog and nosey she wants to be with me all.the time
Baby gates are another useful tool too

Mini schnauzers and the pen I have has a door.  Good point about feeding them in the pen.  That gets them used to it really quickly and they associate it with nice things.

most actually like.them and can go on to use right through their life

Zuma (GSD) is 11 ½ and is currently laying in his We can be anywhere in the house and I can tell him 'in your box' and off he will go to curl up. Only shut the door on it if I am bringing in bags of shopping or similar or if someone who doesn't like dogs has to come in.

Mine got fed in them and were given treats in them to encourage them as nice place to be

Mine comes out again if dogs are poorly and need to be in the bedroom at night.

We also taught him when he was a pup that if he 'pinched' anything and took it in his crate we would do 'swappsey' for a special treat.
 
proves how well that worked ....  about 4 years ago he came rushing in from the garden one evening - into his crate - and put down the very large (unharmed) hedgehog he had 'found'
Told him that it wasn't suitable as a pet for him and gave him his treat then put it back in the garden and it just wombled off.

How lovely such a good boy

How lovely such a good boy

Spoilsport 

I realise puppies probably have to be confined to areas where it's not a big deal re their inevitable mistakes which will happen, early days BUT far better than a play pen would be a crate.  Top and sides covered these make a nice little den which most puppies will use as their sleeping place, door open.   And going forward, being able to be crated will help with car travel/hotel rooms, and recovering patients, probably for the puppy's entire life.   We had our puppies in their crates when we couldn't supervise what was going on, up to around 4 months.   Longer as necessary.

I think play pens have moved on. Here's what we have, not cheap but looks much nicer than metal crate. 
https://www.amazon.co.uk/gp/product/B017D8UNDA/ref=ppx_yo_dt_b_asin_title_o04__o00_s00?ie=UTF8&th=1

We used a play pen with a crate inside for our terrier. So he was crate trained, but also had restricted access to the house (and also, this prevented him from harrassing our old dog). Then the caret got bigger and then it turned into the whole kitchen. He is normally in the kitchen when we're out, but if someone forgets to close the kitchen door he doesn't do any damage to the house. It's only because we don't have a porch that I don't like him running free as people come in and out and don't take much care with the front door sometimes!