Whats the best age to breed a bitch? and how old should the stud dog be? a few people have said 2 but this still seems quite young???

Depends entirely on the breed I'd say. Small toybreeds are okay at 18 months, others 2 years, large breeds personally I prefer 2 ½ years. Using Cavaliers as an example of how it varies between breeds, they are toys but should not be less than 2 ½ in order to a) ensure their parents are at least 5 years old (and heart clear), and b) have been able to be MRI scanned aged no less than 2 ½.

As fr the stud dog, for most breeds it will mean over a year as you cannot hip score until 12 months of age, but for those that do not require hip scoring, I'd still say at the very least a year. Again so much depends on breed. The breed clubs usually have recommendations for their members.

Different breeds mature at different ages and as Marianne says many health tests are carried out at varying ages.

It may be better to think in measures rather than years eg

Has proven to be a good example of the breed by winning consistently in the ring or in field trials
Is physically and emotionally mature
Has certified clear for all required health tests

Then add your own knowledge of the bitch - If you feel she is ready.
Then consider your bank balance at different points of the year- Best to have a good amount available quickly in case of unexpected vet fees etc
The list goes on...

Maybe if you put size of breed (breed not allowed) someone with similar may be able to be more specific

The size of the breed is medium bigger than a terrier but smaller than a bull mastiff.

Is it the dog you were talking about previously in April this year?

Very much depends on the breed.  Mine is 15 inches to the wither, so most other breeds would consider breeding at 2 with this size, but they are very slow maturing and 3 the season closet to the 3rd bday is considered the best for us.  The youngest I have known is 2 1/2 yrs.  Its best to speak to experienced breeders in your breed and get their opinion.
I have also found that if you think your bitch is too immature waiting an extra season (if she is young) is benefitial to you and the dam.  You want the bitch mature enough to rear the pups without you having to step in with her every move.  This however is personal preference, I know others strongly believe that having a litter makes a bitch grow up.  I would rather let my bitch grow up in her own time, but maybe I'm just soft with my babies!