Although it states 6+ on the bottle, drug doses are all calculated by weight - hence the reason for Calpol changing their dosage guidance on the bottle recently.
The 6+ formula will have a diff concentration to the younger formula hence the restrictions. Just like a normal 'adult' paracetamol tend to be 500mg - you could prob look online and find that you could give 1/4 or 1/2 an adult table too (instead of not as well as).
So as a pp said, if your 5yo is about average weight wise then you should be fine.
Here's a dosage guide from www.pharmweb.net/pwmirror/pwy/paracetamol/pharmwebpicdosage.html:
OTC Use
Adults
The recommended dosage of paracetamol in adults is two 500mg tablets (i.e. 1gm paracetamol) every four to six hours, not exceeding eight tablets (4gms) in any 24 hour period (1). This dosage may be continued for several days. If pain relief is required for a longer period it should be with the supervision of a doctor.
Children
Children's dosages vary with the age of the child and the type of product, therefore the instructions on the pack should always be followed.
In general, children's dosages are based on a single dose of 10mg paracetamol per kilogram bodyweight, which can be repeated 4-6 hourly, not exceeding four doses per 24 hours.
On a doctor's recommendation only, paracetamol may be given to a 2 month old child following immunisation as a single dose of 60mg (i.e. 2.5mL paracetamol liquid (oral suspension) at a strength 120mg per 5 mL).
For children under 3 months, on a doctor's advice only, the dosage is 10mg paracetamol per kilogram body weight (5mg/kg if jaundiced).
For a child 3 months to 1 year of age a dose of between 60mg and 120mg (i.e. 2.5mL to 5mL of paracetamol liquid (oral suspension) at a strength of 120mg/5mL) may be repeated every 4-6 hours to a maximum of 4 doses in 24 hours.
For a child 1 to 5 years of age 120mg to 250mg (i.e. 5mL to 10mL of paracetamol liquid (oral suspension) at a strength of 120mg/5mL) may be repeated every 4-6 hours to a maximum of 4 doses in 24 hours.
For a child 6 to 12 years of age 250mg to 500 mg (i.e. 5mL to 10mL paracetamol liquid (oral suspension) at a strength of 250mg/5mL) may be repeated every 4-6 hours to a maximum of 4 doses in 24 hours.