​Outdoor weddings in the summer can be lovely. Be aware, though, that many venues have no shade at all where your guests will be seated.

Your elderly guests will almost always arrive early because it takes them a little while to get to their seat and they don't want to hold up people behind them. For the same reason, they will probably be the last ones out.
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Having older people sit in the direct sunlight for too long is very uncomfortable, and possibly dangerous.  Why not seat older people at the end of a row and delegate a couple of your younger relatives to hold umbrellas over them to shield them from the sun?