Firstly - welcome to the forum
This is an area that causes much confusion and argument especially between vets. There has been much research done into this.
The issues:
If you spay a bitch before her first season the chances of her getting mammary tumours is almost zero. As the bitch has each season the chances of her getting mammary tumours increases, until middle age when there is no protective benefit (a 5 y/o bitch gets no protection).
If you spay a bitch before her first season there is a much higher chance of her suffering urinary incontinence, that will require treatment for life. Having just 1 season significantly reduces the chances of this. (although urinary incontinence is something we see occasionally in spayed bitched)
Around a third of unspayed ******* get a condition called pyometra where the womb becomes infected and full of puss. This requires an emergency spay when the bitch is old and sick and is therefore more expensive and a much higher risk.
My preference is to spay a bitch 3 months after her first season - I see it as a good compromise between reducing the risk of mammary tumours without a great chance of a dog peeing in the house for years!
If you want to know more I will be pleased to ask. Other vets may have different views!