Hi Catrin: as so often with these kinds of questions, it depends! I'd want to ensure this is a good breeder, first off, who breeds for health and also, ensures their puppies are well socialised. Cavalier Matters has good info on the kinds of health documents and information a good breeder should offer up front: see
https://www.cavaliermatters.org/advice-for-puppy-buyers/ and
https://www.cavaliermatters.org/before-you-buy/. And of course, you can ask the breeder why they are homing an older puppy or one year old. For a reputable breeder, the likely reasons are, someone deciding not to take a puppy after booking one, or that the breeder has 'run on' a prospective show quality puppy that turns out not to be a good show candidate. It's hard to see what a pup will be like when they are small but by 5-6 months or a year, it's a lot clearer.
So, if this is a good, health focused breeder, then getting an older puppy or young adult has many advantages. It should be better socialised, perhaps through most of the puppy nipping stage, probably started on housetraining, and likely had the advantage of being around other dogs and people. That said, having a puppy or dog around a four year old means a huge amount of management work to not allow either free access to the other, a setup that allows the puppy/dog to retreat anytime it wishes, and constant child/dog arm's length supervision for any child under 7 ish or so. Just be aware some breeders won't home a pup or adult to a home with children this young.
If this isn't a verifiably good breeder -- and setting aside that of course you'd not want a dog from one anyway -- an older dog of this age, 5 ish months, is likely unsold Christmas stock from a backyard, commercial or puppy farm breeder. An older pup or young adult might be sold due to health or temperament issues they aren't telling you about.
