Although it is impossible to really know how old it is by size, we can guess. How it is kept and health is the biggest factor by far in growth. A leopard tortoise a few months old could definitely be the size yours is with the seams just starting to separate the original scutes. However, judging by the depth and look of the seams, it looks to me that tortoise has been kept quite dry and very slow growing. It could be a year old with seams that deep with the keratin looking like it has had time to ridge up and "plateau" the original scutes like that.
Bottom line - we don't know. My guess - 8 mos to 1 yr.
By the size, it compares to a youngster, hatchling age but like Mark said, depending on how it was cared for it could be close to a year. That's a cute little leopard!