What matters (wishing I had a punctus) far more than age or mode of communication, is context. And that is what we need to teach kids today — what the 21-year-old who commented displays. If I’m texting a client, I use proper grammar and full sentences with periods. If I’m texting a friend? Not so much. And if I’m texting my taciturn and familiar brother, I’ll probably just use emoji (or a line from a movie) with no other explanation or punctuation needed.
While each medium affords different uses (texting lends itself to less formality than email), the primary consideration is context, even when speaking. A parent rarely speaks to their child in complete, formal sentences (unless said child is in serious trouble), so why would they text their child that way?
Confession: In jr high, I got in trouble with my friends for passing notes with full sentences!