Socialising should start as early as possible in a dogs life. Puppies go through a critical socialising period between 3 and 17 weeks of age. During this time - the puppies experience will shape and influence their adult behaviour. They should be exposed to lots of different situations - people, dogs, animals, vehicles, noises and more.
A dog is never too old to learn 'new tricks' but the more they get 'set in their ways' the harder it is to correct. Some owners simply adapt to their dogs behaviour while others do not. If you or your dog are unhappy and you are truly committed to overcoming the problem there is usually a way. I say 'usually' and not 'always' as some dogs may do better in some situations with a different handler/owner - not all owners for example have the ability to change a part of their behaviour that might hold them back from leading their dog to their full potential.
Some dogs simply do not like other dogs. Whatever your dogs reason for not liking a particular dog might not be clear and sometimes personalities will just clash. If your dog is particularly fearful or has had a bad experience - socialising can be immensely stressful for dog and owner. I am a firm believer of 'quality not quantity' when it comes to choosing dog friendships.
Should I take my puppy to socialising classes?
If you can find a good one then yes - but it is by no means essential. The best way for puppies to socialise is for them to mix in a controlled environment with a mixture of dogs - ages, sizes and temperaments (all friendly dogs but some more playful than others) . Puppies can only learn very basic socialising skills from interacting with other puppies. One way to explain the limitations was put well by a fellow dog trainer 'It's like putting two toddlers in a room together. Yes they'll act like toddlers and will not earn anything other than how to be a toddler. Give them a book on statistics and they will not teach each other statistics - they need a more experienced person to show them how'. (ok maybe using statistics as an example is a bit over the top but they message is the same - the ins-and-outs of socialising needs to come from someone more experienced)
Many puppies do not suit 'puppy parties'. Most involve one or two big or noisy puppies who jump on and scare the rest of the 5 already nervous puppies who then proceed to hide under their owners chairs for the rest of the session. Puppy parties like this (especially those that are held in vets) are in my opinion the worst way to 'socialise' your puppy. Not just does your puppy learn that they should fear other dogs but also that this environment (the vets or training hall) is a scary environment. I have met only a handful of dogs who enjoyed or learnt anything from 'puppy parties' and the rest either found them no use and usually detrimental. Pick your 'party' carefully and ask who and what will be attending. Do not fear large dogs if you have a small dog - just be sure the socialising is controlled by an experienced organiser.
Some puppies may enjoy puppy parties if they are well managed by owners and organisers. More importantly - owners should continue to expand their dogs exposure when out on walks, in the car, at a friends house and wherever they go.