Two overlapping criteria
Cutting describes where the material came from. Young plant describes its stage. A rooted cutting can be both. A seedling can also be a young plant and started from seed.
What to record for a fair comparison
Note the developmental stage and documented origin. ‘Rooted cutting, batch A’ carries more information than ‘young plant’. A seed lot may be the relevant reference for plants grown from seed.
Example: asking what age means
If two plants are described as the same age, ask what event started the clock. Sowing a seed and taking a cutting are different events. A dated observation helps compare the actual developmental stage.