The RPL21P99 gene is a member of the ribosomal protein gene family and is classified as a pseudogene. The designation "RPL21" indicates that it is related to the RPL21 ribosomal protein, which is a part of the large ribosomal subunit. As a pseudogene, RPL21P99 typically is not translated into a protein, but it may have regulatory or other functions. 

### Additional Information:

1. **Associated Genes**:
   - Given that RPL21P99 is a pseudogene related to ribosomal protein genes, it may share associations with other genes involved in ribosome assembly or function. However, detailed information about specific associated genes not mentioned in public databases can vary. Commonly associated genes could include others from the ribosomal protein family, such as RPL21, RPL22, and RPL23. 

2. **Aging and Age-Related Diseases**:
   - While the direct implications of RPL21P99 in aging or age-related diseases are not explicitly documented given its status as a pseudogene, ribosomal proteins and their regulatory mechanisms are integral to cellular functions such as protein synthesis, which is known to decline with age. This decline impacts cellular metabolism and can contribute to senescence and age-related diseases. Some studies suggest that changes in ribosomal biogenesis may correlate with age-related phenotypes, although a specific connection to RPL21P99 would require further investigation.

3. **Drug Interactions**:
   - As of the current knowledge, there are no specific drugs or drug classes directly known to interact with the RPL21P99 gene. However, since it is related to ribosomal function, drugs that target ribosomal proteins or ribosomal functions (such as certain antibiotics) might indirectly affect pathways that include this gene.

4. **Pathways and Biological Processes**:
   - RPL21P99, being a pseudogene related to ribosomal protein genes, is theoretically associated with pathways involving ribosome biogenesis and protein synthesis. Other biological processes that could be influenced by the presence of RPL21P99, either directly or indirectly, include:
     - Regulation of cellular metabolism.
     - Cellular response to stress.
     - Protein targeting and localization.
     - Developmental processes related to cellular growth and proliferation.

### Conclusion:
While RPL21P99 may not exhibit significant functional characteristics due to its status as a pseudogene, understanding its relationships with other ribosomal proteins and involvement in essential biological processes highlights the importance of ribosomal structures in fundamental cellular activities and potential implications in aging and disease. Further research is needed to delineate any specific roles it may have, especially in the contexts mentioned.