Learn Portuguese (Portugal): A banheira é velha mas boa.

We use é, which comes from the verb ser, to talk about permanent or defining characteristics. The bathtub being old and good are basic facts about what it is, rather than a temporary state or condition (where we would use estar).

Why do the adjectives end in "-a" (velha, boa)?

In Portuguese, adjectives have to match the gender of the noun they describe. Because banheira (bathtub) is a feminine noun, the adjectives change from their default masculine forms (velho, bom) to their feminine forms (velha, boa).

Can I use velha to say a person is old, just like the bathtub?

You can, but it often sounds rude or disrespectful when talking about people. For objects like the bathtub, velha is exactly the right word. When talking about people, it is much more polite to use idosa (elderly) or mais velha (older).