As stated here :
(in English) "The voiceless stops /p/, /t/, /k/ are typically aspirated when they begin a stressed syllable, becoming [pʰ], [tʰ], [kʰ] [...]"
Since these consonants weren't aspirated in Old English, I thought the aspiration would appear and be explained in a diachronic description of English, like this one. As far as I know, the explanation is missing.
Hence my questions : (1) How and when the aspiration appears ? (2) From when is the aspiration securely attested ?
addendum : I read in Hogg's Cambridge History of the English Language (tome I, p. 89-90) :
"It is impossible to be more precise about the articulation of the [Old English] dental consonants [...] there is no way of telling whether these stops would have aspiration in initial position [...]"