The answer is
spring
Great thanks to jafe and Brandon_J
Let us begin at the beginning
spring --> beginning of the year
With all my mates and endless spinning
other seasons, rotating every year
I'm not so hot and not so cold
spring is a season where the temperature is moderate
I'm the sea and the sun, and quite as old
nice one! sea+sun sounds like season, where spring is one of them
When shove comes to push
when force is applied
In your hand, I'm worth more than a bush
Day in, day out, hidden talents
Up and down, I keep the balance
spring balance
I give birth to the flow
spring as in water flow
Downhill my children go
a stream of water usually flows downhill
Through me you can see clearly
we can see through water clearly
Now name me, riddler, dearly
spring
Why posting this riddle in this time of the year?
Spring Equinox?