When told a "developmental age of ○ years," should I interact based on their actual age or developmental age?

The knack is to keep both in mind rather than only one. Matching the difficulty of understanding and tasks to the developmental age lets them engage without strain. Meanwhile, respect the actual age for interests and how they're treated (word choice and respect). For example, even with easier materials, choose content and design suited to the child's age. Rather than deciding "at ○ years it's like this," start from the child's own "can-dos and likes" in front of you.