Open the Horse's Thoracic Cage

as a vertebra out of alignment your horse can be very grumpy under saddle which is understandable. Third the growth plates in the vertebrae and ribs fuse close to last usually between five to seven years of age so there should be no rush to start your youngsters especially under saddle.
Kissing spine syndrome also known as spinous process impingement is where the long upward projecting spinous processes are touching. Most cases of kissing spine are seen between T13 and T18 directly under where the rider sits. The horse may exhibit symptoms including anxiety discomfort during grooming difficulty mounting back stiffness unwillingness to go in collection and a loss of muscle mass over the topline. If you suspect this might be a problem for your horse contact your veterinarian for a thorough exam and xrays.
The Rider
As a rider you may underestimate the impact of your own thoracic spine mobility on your effectiveness in the saddle. Stiffness in this area can restrict the easeful movement of your spine including the sacrum and pelvis can increase tension or mobility in shoulders or arms can create a change in your lumbar or cervical curve or can lead to asymmetry. In response our horses may need to compensate or even act out to address our immobility which is not comfortable for them.
To use myself as an example the rigidity in my thoracic spine and cage was a major contributing factor to issues within my lumbar and cervical spines pain in my sacroiliac joint and even migraines. The more open I was in this region the more able I was to give subtle and independent aids with my body position and have clear communication with my horse.
Working Together
The best and most lasting changes to my riding have occurred with regular osteopathy sessions. I recommend that every horse and rider work with their favourite manual therapist on restoring resilience and functionality to this area and include exercises to mobilize both of their thoracic cages in their regular routine. 
I love working with classical training on the ground my favourite local coach is Heather Nelson who has trained in the Academic Art of Riding to help horses become freer and more symmetrical through the thoracic cage and improve their ability to lift up and through and power from the hindquarters. Personally I work daily with thoracic rotation cat pose and downward dog pose and optimally attend a dance
or yoga class every week.