Just had the same problem (mid negotiation the landlord tells us that he decided not to have children in the house) and so I did some research. this is what I found (the Equality Act 2010):
"What kind of behaviour can be unlawful discrimination?
The Equality Act says the following things can be unlawful discrimination by someone who has the right to sell or let a property, if it's because of who you are:
offering you a property on worse or less favourable terms
refusing to let or sell you a property
treating you badly or less favourably when you want to rent or buy a property
behaving in a way which causes you distress or offends or intimidates you - the Equality Act calls this harassment
punishing you because you complain about discrimination, or help someone else complain - the Equality Act calls this victimisation.
What’s meant by age?
If you want to make a discrimination claim, you need to find out if you’re someone who mustn’t be discriminated against under the Equality Act 2010.
The Equality Act says it’s only unlawful discrimination if you’re treated unfairly because of certain reasons. These reasons are called protected characteristics. Age is one of the protected characteristics under the Equality Act.
Age discrimination is where you’re treated unfairly because of your age or because you’re part of a particular age group.
What’s meant by age group?
Age group means people of the same age or people in a particular age range.
Here are some examples of age groups.
You’re 25 years old. You could belong to one of the following age groups:
25 year olds
under 30s
over 20s
people in their 20’s
young adults.
You’re 78 years old. You could belong to one of the following age groups:
78 year olds
over 70s
pensioners
senior citizens.
Example
You’re 70 and you’ve been refused entry to a gym because of your age. This is unlawful discrimination.
Discrimination because of someone else’s age
It’s unlawful to discriminate against you because of the age of someone you’re with or someone you know. This could be a parent, child, partner or friend.
This is called discrimination by association.
Example
You’ve booked a holiday for you and your mum, who’s in her sixties, to go on an organised walking trip to Wales. When the organisers find out your mum’s age they say you can’t go because your mum is too old to keep up with the rest of the group. This could be unlawful discrimination against you and your mum because of her age."
www.adviceguide.org.uk/wales/housing_w/discrimination_housing_e/housing_identrifying_discrimination_e/hp4_what_is_the_unfair_treatment_when_renting_and_buying_.htm
www.adviceguide.org.uk/england/discrimination_e/discrimination_protected_characteristics_e/ge1_age_discrimination.htm