The OtoScreen device comes in two varieties: "Pediatric" and "Standard." The Pediatric version tests hearing at 20 dB HL and the Standard version tests hearing at 25 dB HL.
Young children normally have better hearing than the general population and is why they should be tested at the softer 20 dB HL level. For testing in K12 schools, some States require that the 20 dB HL level is used, others don't or require the 25 db HL level be used . If you intend to use the otoscreener for school testing you should check your State's regulations before you buy.
Most of the OtoScreen units sold are Standard.
Can you test adults with a Pediatric OtoScreen unit? Of course. However, there is a chance of a "false negative" result in the case where the adult can't hear the 20 dB HL tone but would be able to hear a 25 dB HL tone.
Can you test children with a Standard OtoScreen unit? Of course. However, there is a chance of a "false positive" result in the case where the young child can hear the 25 dB HL tone but would not be able to hear a 20 dB HL tone.
Although the "false negative" and "false positive" possibilities mentioned above would be few in number (statistically) you should still consider this in making your decision of which type of OtoScreen unit to buy.
A "rule of thumb" is, if most of your subjects are going to be young childen buy the Pediatric version, if not, buy the Standard version. Or, buy both versions to cover both cases.