Actually when the plants are young most of phenotypic variability is due to a strong environment impact. As the plants grows, genes express over time and increase the accumulative phenotypic variability due to genetic factors, and as the plant get stronger, it displays more resilience against environmental factors, which explains why heritability increases as the plants grow.
The trait measurement in different ages represents different traits, for example, birth body weight, weaning body weight and adult body weight represent different traits and have different heritabilities.