Learn Portuguese (Portugal): As escadas do prédio são velhas.

São comes from the verb ser, which is used for permanent or inherent characteristics. The physical age of the stairs is a lasting trait of the building, not a temporary condition, so we use ser rather than estar.

Why is the adjective velhas plural and feminine?

In Portuguese, adjectives must match the noun they describe in both gender and number. Because velhas describes as escadas (a feminine, plural noun), the base word velho changes to its feminine plural form, velhas.

How does the possession work here without an apostrophe 's'?

To say "the building's stairs" in Portuguese, you have to flip the order to "the stairs of the building". We use the word de (of) combined with the word o (the masculine "the" for prédio) to make the contraction do. So, as escadas do prédio literally means "the stairs of the building".

Could I just say de o prédio instead of do prédio?