[1][2] Born in Llantrisant and raised in Pencoed,[3] Costelow spent time with the Ospreys youth teams before moving on to Leicester Tigers in 2018.
[4] Having twice won the Academies League,[5] Costelow earned a place with the Tigers' development squad and went on to make a loan move to Championship side Ampthill.
[12] A Wales youth international at under-16 and under-18 level, Costelow stood out during the 2020 Six Nations Under 20s Championship putting in a Man of the Match performance against England.
[14] Following a late team change, Costelow was named on the bench against New Zealand on 5 November 2022, coming on in the second half to make his test debut.
[16] In November 2024, Costelow replaced the injured Mason Grady on the wing for Wales 24-19 2024 Autumn Nations Series loss against Fiji.