In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The prevalence of repeatedly low testosterone increased with age ( 4 % before age 50 years and 9 % 50 years or older ) .
### Output Text:
The prevalence of repeatedly low <span class="chemical">testosterone</span> increased with age ( 4 % before age 50 years and 9 % 50 years or older ) . 