I'm not sure which of the following statements is most appropriate in case of a seedling:
Three year old seedling Three-year-old seedling 3-yr-old seedling
The third one seems to be a nice abbreviation, but I'm not sure if it's a good fit for a plant's description which is going to be used within an article.
Could you please explain what are the differences between them? I'd like to use the third one, is it appropriate for an article about plants (which should sound official)?