I can understand ADM's frustrations, and I can't say that on a given day that I wouldn't have behaved the same way. But, as someone who was in the shop to witness these events, these are the facts.
There was a smallish queue, with around a 10-15 wait time. As ADM said, one of the two club staff was dealing with season tickets, the other with merchandise. You might argue that selling tickets should be a priority, and I would agree. But given the arrangements, it would have simply meant a few minutes additional wait for the chaps before him to be dealt with by the lady selling the season tickets.
As it was, the lady who was selling the merchandise agreed to log on in order to book the STs. She asked the age of his daughter and pointed out that, unfortunately, the daughter wouldn't be eligible for a junior ticket and, following the protestations, politely pointed that as unfortunate as it was their had to be a cut-off date.
It would have been incredibly frustrating for ADM - and I've been there and done that. But to me it looked like a case of a customer team rep doing their job in a professional manner.