From about what age, and in what words, should I talk about it?

There's no set age; a natural time is when the child starts to notice differences, or when they ask. Around the start of school, begin with concrete, positive words like "strengths and weaknesses" and "everyone is different." At school age, explain the diagnosis or "a difference in how the brain works," linked to the child's own experience ("sounds seem loud," "it takes time to remember"). Words tied to what's actually happening in their life get through better than abstract explanations. Picture books and first-person accounts help, and share the outlook that "when you're stuck, there are ways to make it easier," too.