The thing is, each kid has his or her own timeline for when they will accomplish things. Juliet has started walking at 21 months. Other kids are earlier. Some are later. The same is true for every developmental milestone.
When people know the general ages that children typically meet milestones, they ask parents, "Is your little buttercup walking, talking, potty trained, solving quadratic equations yet?" Of course their intentions are to strike up a nice conversation. But when a child is not meeting milestones at the same rate as their peers, it can be stressful for parents.
Yet, these same parents do have wonderful things that they want to share about their children. Even children with significant impairments have wonderful traits and abilities that parents love to talk about. I have found the best way to start conversations without causing unnecessary awkwardness is to ask a more open ended question.
"What are Little Susie's latest tricks?" or "What great things has Little Johnny done lately?"
This allows the parents to talk about what their child can do rather than focus on what they might not be able to do. Maybe a parent will talk about a new food their child has tried. Maybe they can't walk, but love playing peek-a-boo. Maybe they haven't ever slept through the night but have a crazy-funny laugh.
Most people (at least the ones that aren't incredibly annoying) don't want to make parenting into a competition. They want to support their friends and help them along this journey. They want to celebrate children's accomplishments. You may not know if a child is having struggles - not all challenges are visible. Asking open-ended parenting questions is a good way to start some good friendly conversations.