If the Sun has access to your fence you will want to seal it.
The Sun takes out the natural oil that is in the wood. Once all the oil is gone from the wood then the integrity of the fence (or deck) is compromised. You know this has happened when you grab a part of the fence and it crumbles In your hand. Then all the oil is gone and you have waited too long, the only thing you can do at that point is replace the fence.
If you grab the fence with your hand and the wood is still solid, then the wood might have lost some oil but its integrity is still intact.
If you seal the fence now the sealant replaces the lost oil the sun has taken out and then closes the pores of the wood which stops any further deterioration. Sealing the wood can make a fence or deck last 2 to 5 times longer than it would unsealed and unprotected.