I've observed and heard many parents say about their (post-) b'nei mitzvah child that they notice newfound confidence. It's a spill-over effect, which generates improvement in many other areas of the child's life. Parents tell me their child's public schoolwork improves...Some say peer or sibling relations turn a corner in the direction of maturity
I wonder if we, in the Reform Movement, are really using this newfound maturity to best advantage.
To me, this is a wide-open window and families should be encouraged (by clergy, religious school teachers, Ritual Committee communication) to let the child do some more mitzvot in the community, since now he/she can.
Personally, I'd like to see teens brought around more to shivah homes alongside their parents, sometime in the year or two following B'nei Mitzvah. B'rit Milah is often during school-time, and weddings today don't seem to involve participants except as "guests," but shivah house visits, or even a light bedside hospital visit might express the child's new level of communal responsibility. To me, this is how one "affirms" a child deeply, as follow up after Bar/Bat Mitzvah beyond what my cousin called aptly, "the sludge of thank-you notes." It's not always about getting attention on the bimah...being counted in a minyan in a home would be huge to many teenagers.
It's the parent's chance to model many behaviors: How to console by listening; how to bring in food without fanfare; how not to expect normative hospitality (coat-hanging services, etc.) from the mourners, and so on. When the child's own home becomes a shivah house, someday, they'll not be so caught off-guard by all the customs.
My son, who attended a rural Orthodox day school full of modest, kind people, said his Orthodox friends made less of the Bar Mitzvah service (just the maftir--sneeze and you miss it!) but much more of the tasks expected/required once majority is reached. I find that admirable.
The deepest way to "affirm" a child is to ask more and structure new areas of communal responsibility of them, particularly in the post-b'nei mitzvah year when the window of the soul is so open to feel the before/after difference.