>>> The maximum sentence for criminal damage is three months in custody
That's a bot of an over-simplification. The maximum sentence for criminal damage (contrary to s.1(1) Criminal Damage Act 1971) is nominally ten year's imprisonment. (i.e. that's the maximum penalty provided for within the actual legislation). However the maximum penalty actually available to the courts is one of 4 years imprisonment but only where the damage is valued at more than £5,000. It's only when the value of the damage is assessed at no more than £5,000 that the maximum penalty is limited to 3 months of custody. So don't go smashing any very expensive windows!
(There might not be many buildings with windows costing more than £5,000 to replace but I certainly know of at least one).
Courts aren't meant to take the age of an offender into account when determining the sentence for an offence (other than in the case of youth offenders). So, whether you're in your early 50s, your early 20s or your early 90s should make no difference.
For breaking a small window, the 'starting point' sentence is a Band B fine, with magistrates then looking at whether to move that sentence down to as low as a conditional discharge or as high as a Band C fine.
For breaking a large plate-glass or shop window, the starting point is a low level community order, with magistrates allowed to drop that to a Band C fine or raise it to a high level community order.
(A 'Band B' fine is between 75% and 125% of 'relevant weekly income'. A 'Band C' fine is 125% and 175% of such income. Community orders typically refer to periods of unpaid work).
When deciding whether to move a sentence up or down from its 'starting point', magistrates have to take into account the 'level of culpability' involved in the offence. 'Provocation' (which might be relevant in your theoretical case) can push a sentence downwards but a 'revenge attack' (which might also be seen as relevant) can push it upwards. Magistrates also have some discretion to reduce a sentence for a guilty plea.