get into shirts

[tree123]

All were looking so safe and cosy that she smiled at her fears now and sat down tranquilly by the fire to sew.

It was something for Michael, who on his birthday was getting into shirts. The fire was warm, however, and the nursery dimly lit by three night-lights, and presently the sewing lay on Mrs. Darling’s lap.

Source: Peter Pan by J. M. Barrie

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Is 'get into shirts' idiomatic? I think this refers to Mrs Darling was sewing a shirt for Michael as a birthday present. The expression is very strange to me.

Should 'shirts' be in plural?

 

[tree123]

It's possible that Michael had been wearing dresses up to this stage in hislife. On his birthday, he was going to be breeched, at which time he would also start to wear shirts.

Interesting.

Wikipedia is blocked from here, but I've searched the relevant info of 'breeching' from other website.

 

[emsr2d2]

Interesting.

Wikipedia is blocked [STRIKE]from[/STRIKE] here, but I've searched for the relevant info [STRIKE]of[/STRIKE] on 'breeching' [STRIKE]from[/STRIKE] on another website.

Note my corrections above.