The applicant must be 16 or over to apply for a student visa. The maximum age is not defined.

Note:

On behalf of the work as an au pair the minimum age is 18 for

"(j)... under 18 years of age, the applicant’s parents or  legal guardian ... must confirm that they consent to the  arrangements for the applicant’s travel to, and reception and care in,  the UK."

(Pls see Immigration Rules, paragraphs 245ZV - 245ZY).

17 to 30

18 to 30

When applying for a student visa, one needs to prove one's knowledge of the English language when applying.

This usually means passing a secure English language test (SELT).

Refer to Tier 4 visa knowledge of English

Please note that the test needs to be from an approved provider.

An au pair can prove one's knowledge of English by passing an approved English language test with at least CEFR level B1 in reading, writing, speaking and listening

Some host families may want to have an au pair who has first aid training, but that is a preference, not a requirement.

 Au pairs are NOT a substitute for qualified nursing assistance for families who require that service for themselves or children .

There are no restrictions found on the marital status regarding the work as an au pair.

In generally your family members (dependents) might be able to apply, to join or remain with you in the UK if they’re from outside the European Economic Area (EEA) or Switzerland.

Regarding the work as an au pair there have been found no restsrictions about the au pair being childless or not.

But:

Student visa holders may apply to bring a dependent to the UK if they’re:

• sponsored by a higher education institution on a course at level 7 on the Ofqual register or above that lasts 1 year or more
• a new government-sponsored student on a course that lasts longer than 6 months
• a Doctorate Extension Scheme student

Recommendation:

In case the au pair wants to bring a dependent to the UK to stay with him / her that should be negotiated with the host family.

The au pair cannot apply for a visa if he or she has:

• children who live with him or her
• children he or she is financially responsible for

this is stipulated at this U.K government site

• Two parents, married or unmarried, and their dependent children, .
•  A single parent or other adult carer/guardian with dependent children.
• An extended family unit that may include, for example, one or two parents,grandparent(s), aunt(s), uncle(s) – or any combination of extended family members - with dependent children.
• A same sex couple, with dependent children.

• In general, the host family speaks the local language in order to best acclimate the au pair to their new environment.
• While au pairs often help teach children words or phrases from their own language, it is English that is primarily spoken in the household to provide consistency.
• Many au pairs wish to work in the U.K in hopes of improving their English skills.

Generally at least 1 member of the host family has U.K. Citizenship

Find more information for au pairs and host families on the British Au Pair Agencies Association website. This link is from Gov.UK

An au pair must register with Ofsted or a childminder agency if the au pair wants to be paid to look after children under 8 for more than 2 hours a day in England - unless you’re exempt.

There are 2 registers. 

• the Early Years Register to look after children aged 5 and under
• the Childcare Register to look after children aged 5 to 7
• both registers to look after children of all ages

The Early Years Register is for children from birth up to the 31 August after their fifth birthday. An au pair must be on the Childcare Register to look after children after that, up to their eighth birthday.

An au pair can get an unlimited fine or be sent to prison or both if childminding or daycare services are provided without being registered.

For more information about the process of registering process with Ofsted or a childminder agency please visit the homepage of Gov.UK, section "Register as a childminder or childcare provider (England)".

Important:

There are different rules for Scotland, Wales and Northern Ireland.

Note:

The registering usually takes up to:

• 12 weeks for childminders and home childcarers

So registering should be done before starting to work as an au pair.