Parashas Bamidbar
מִבֶּ֨ן שְׁלֹשִׁ֤ים שָׁנָה֙ וָמַ֔עְלָה וְעַ֖ד בֶּן־חֲמִשִּׁ֣ים שָׁנָ֑ה כׇּל־בָּא֙ לַצָּבָ֔א לַעֲשׂ֥וֹת מְלָאכָ֖ה בְּאֹ֥הֶל מוֹעֵֽד׃
from the age of thirty years up to the age of fifty, all who are subject to service, to perform tasks for the Tent of Meeting.
Torah Temimah points to TT / Chullin related (verified).
מבן שלשים שנה — ולהלן הוא אומר מבן חמש ועשרים שנה, הא כיצד? כ״ה ללימוד ושלשים לעבודה.
English: “From thirty years old” — and elsewhere it says from twenty-five years old; how is this? Twenty-five for training, thirty for service.
Custom
Chazal reconcile the two ages: five years of training from 25, full service from 30.