Here's why:
* Individual variation: Neon tetras can grow at slightly different rates, even within the same tank. Some may grow faster than others due to genetics, diet, or tank conditions.
* Lack of distinct aging features: Neon tetras don't have any specific physical changes (like rings on a tree trunk) that indicate their age.
* Short lifespan: Neon tetras generally only live for 2-4 years in captivity. This means they don't have enough time to develop significant age-related changes.
What you can do:
* Keep good records: If you purchase a neon tetra as a juvenile, you can estimate its age based on how long you've had it.
* Observe its health: Healthy neon tetras generally live longer, but this is not a foolproof method.
* Consider its size: While not an accurate indicator, a larger neon tetra might be older than a smaller one.
It's important to remember that even if you can estimate a neon tetra's age, it's not a guarantee of its longevity. Like all fish, they can be susceptible to various health problems.