the oldest is an adjective, which is a superlative of 'old'. It isused to describe the age or seniority of something. It is used similarly to other adjectives denoting age and is paired with a noun to describe a quality of the noun in question.
1. The oldest house on our block has been around for more than fifty years.
2. Our pet dog is the oldest pet in the home.
3. We have the oldest model of this technology in our lab.
when using 'oldest' as an adjective, be sure to use it before any other adjectives in the sentence. Otherwise, the meaning of what is being conveyed may become unclear. Additionally, do not make the mistake of using the comparative form 'older', as the adjective 'oldest' already implies comparison.