The RPL21P129 gene is classified as a pseudogene of RPL21, which encodes a ribosomal protein that is a component of the ribosome responsible for protein synthesis. Here’s a detailed summary addressing your requests:

### Gene Summary:
- **Gene Symbol**: RPL21P129
- **Type**: Pseudogene
- **Location**: Chromosomal location can vary based on genomic region annotations and may involve specific contexts in human genetics.

### 1. Associated Genes:
While RPL21P129 itself is a pseudogene, other genes that might be connected to it mainly include functional relatives and those involved in the ribosomal pathway, specifically:
- **RPL21**: The functional counterpart providing the basis for the pseudogene.
- **Other Ribosomal Protein Genes**: Such as RPL22, RPL23, RPL24, etc., could be considered associated based on structural and functional genomic contexts as they all play roles in ribosomal assembly and function.

### 2. Aging and Age-Related Diseases:
Currently, there is limited direct research linking RPL21P129 to aging or age-related diseases. However, since it is a pseudogene related to ribosomal function, it could indirectly influence cellular aging or behavioral responses linked to ribosome biogenesis and protein homeostasis. Disruption in ribosomal protein synthesis or function has been associated with certain age-related conditions like:
- **Cancer**: Anomalies in ribosomal proteins have been linked with oncogenesis.
- **Neurodegenerative Diseases**: Impaired protein synthesis can affect cellular functions related to neurodegeneration (e.g. Alzheimer's disease), though specific studies on RPL21P129 are scarce.

### 3. Drug Interactions:
Being a pseudogene, direct drug interactions with RPL21P129 are not established. However, drugs that impact ribosome biogenesis or target pathways involving ribosomal assembly may have indirect effects:
- **Antibiotics targeting ribosomes**: Such as aminoglycosides (e.g., Gentamicin) or macrolides (e.g., Azithromycin), while they primarily target microbial ribosomes, represent classes of drugs that interact with ribosomal processes.

### 4. Pathways and Biological Processes:
Given its nature as a pseudogene, RPL21P129 does not have defined pathways or biological processes attributed to it. Nonetheless:
- **Ribosome Biogenesis**: Any involvement with related genes suggests a role in the assembly and maintenance of ribosomes.
- **Cellular Growth and Division**: As part of the larger network of ribosomal protein genes, it contributes to general protein synthesis vital for cell cycle regulation and normal cellular operations.

### Conclusion:
RPL21P129 acts primarily as a remnant of the active gene RPL21. Its status as a pseudogene means it does not have a direct biological role or pathway, nor does it appear to play a significant role in aging or drug interactions based on the current literature. However, examining the entire ribosomal protein gene family including RPL21 may provide insights into aging processes and disease pathways relevant to protein synthesis.

Continued exploration of the roles of ribosomal proteins and their pseudogenes in cellular aging and disease may yield valuable insight in the future.