When war broke out on 3rd September 1939 conscription age for men was 18 - 41. In December 1941 this was changed to 18 - 51.

The youngest soldiers to be conscripted in WW1 (officially at least) would have been 19 in 1918, therefore they'd be 39 in 1939 and thus called up for active service again.

So, between 1939 and 1941 men in their mid 40s would have likely joined the home guard, by 1942 the only men left were in their 50s (ignoring reserved occupations/medical exceptions etc).

WW1 veterans probably did serve in the home guard, but at best they were 40 years old. Obviously there were underage soldiers in WW1 who lied about their age to sign up, even so they'd be in their late 30s by 1939.

Its just the way the numbers work out.