Aged sake is very rich and deep umami with hint of nutty flavors like sherry. In Japan, you can find sake which has been aged for 25 years.
When sake is first produced, it is as clear as water. However, as it ages, the color gradually deepens to amber. This color is not due to the storage method (such as in a barrel), but from the Maillard reaction, in which dark melanoidin (which gives beer its golden color) affects not only the color but the taste as well. This process is also called an aminocarbonyl reaction.
This reaction occurs after glucose and amino acids are combined and is also observed when bread is toasted.